Well, if you left it in a common space, i get him being upset. But - how bad was what you wrote? Was it down and dirty, your worst thoughts vented (name calling, "BM is a sl&tty waste of skin and her kid is ugly and stupid", etc.), or did it talk about behaviors you don't like? If it was the name calling, apologize. If it dealt with how you feel about certain behaviors that you truly believe are wrong, apologize for leaving it out but own the thoughts inside it. These things have been bothering you but you didn't feel empowered to say them out loud and seek resolution. Now, you can.

Also, as far as you leaving it out - did she find it because she was there outside a scheduled time and you weren't notified she was coming? If so, your SO has no room to bitch. 

And nothing can be done except to let the peices fall where they will. You can try to explain to your BF this was just venting and feelings "in the moment" but depending on what you wrote.....it may not suffice.

Unfortunately, you are going to have to take accountability in this issue. You should NOT have left something so private and confidential just laying around, knowing your BFs kids come to your home. Yes, she should not have read your private journal  but this is the Step world and anything we say/write "negative" about this life MUST be protected by lock and key, alias screen names (as we do on this site) or it WILL come back to bite us. 

I know journaling can be very healing and theraputic....it's one thing I do NOT do because I don't want people "finding" my writings....like what's happened here. So, I just keep my thoughts in my head, with my counselor and with my trusted friends.

Hoping a good outcome for you.

I'd tell him better that I journaled than verbalizing it.  I would hold him to some accountability for raising the little darling.   It's going to be awkward for a bit but hopefully some change will come along soon.  Addressing the reason for the journal in the first place needs to happen.  

Burn it, and going forward when you start a new journal, keep it in a biolocked drawer safe in your nightstand.

Then stand on "I never said those things about them". You didn't say it, you wrote it.

Play the semantics and going forward keep your journal locked up.

Bad move leaving it out.  

To mitigate the risk even more, just in case you do leave it out, make it a lockable journal with a flap with a lock. Or, must write in cursive since the spawn likely are too stupid to know how to read actual handwriting.

As for your SO, go on the offensive, when he spouts his anger the hurt fee fees of his spawn, restate your position referencing absolute fact and demand that he justify their toxic bullshit because they did those things, you know it, and he knows it.  As for your journal vents about BM, make sure he knows damned good and well that she is the POS you vented to your journal that she is.

If this is a never ending hurt fee fee point, call a locksmith, re-key the locks, put hurt fee fee daddy and his hurt fee fee spawn on the lawn and get on with living YOUR best life.  They can run to mommy's place and comiserate on how hurt their fee fees are. Good riddance if that is what you choose to do. I would btw.  They violated a trust, you didn't.  Your journal is yours. Not theirs. No matter where it is located. Daddy seems to have his head up his ass on that truth.

But, you have learned that your journal is only confidential if you keep it confidential.

Just my thoughts of course.

You shouldn't have left your journal in a public space, but neither should your SD have picked it up and read it. These are your private thoughts and not for public consumption - I always kept my journals locked away when the SDs were staying. But hey, novelist Anne Lamott said "you own everything that happened to you. Tell your stories.  If people wanted you to write warmly about them, they should've behaved better." 

I get that you would hope for some privacy.. but TBH.. a juicy journal read would be awfully difficult to pass up.  And... your husband is right to be upset that you left something that had such loaded content in a public space.   It's not like your SD went rifling through your lingerie drawer to find it... she could have just picked it up.. maybe looking for a piece of paper or something.. and when she opened it.. she immediately saw things written about herself and mom.. so read further.  I would admit that if I came accross something like that.. my curiosity would probably override the decency to not read it.... especially if it was right out in the open so to speak.

I am not sure if your relationship with her has been surface civil or whether there has been a lot of strife.  I think that you should tell your husband that while you are sorry that you left it where it could be found, you needed to vent these feelings because holding them in would not be healthy.  That while it may not have been nice for her to see what you thought at times, that his daughter and mother were not nice to you either.. and he should have some amount of irritation at his daughter for reading something that was clearly meant to be private.

Re the daughter.  I think the  message is this.  I'm sorry you may have been hurt by reading my private journal.  Unfortunately, the way I vented about things that frustrated me were obviously not meant to be things aired publicly and were not filtered in any way.  You also should understand that while we all may vent and blow off steam in a frustrated moment.. that doesn't mean that is the totality of our feelings towards a person.  I can admit that I have had my frustrations with you over the years and your mother has done many things that have made my life difficult... so while I'm not happy you saw those private vents, I'm sure you have had similar feelings about others.. even me.. whether you wrote them down or not.  Again, I apologize for hurting your feelings but the intent of those writings was to purge the negativity from my mind.. not hurt anyone else.

Reading ,and touching , other people property.   And you are to blame because you didn't buy a safe and hire armed gards to protect it.  This is bull shi$.  SD gets a pass , BM sticks her nose into a place she shouldn't   And it's all your fault.  

'I think you need a serious look at this relationship,  BF doesn't have your back.  Your personal stuff should not be touched by anyone.  SD would not like you going through her phone if left on a table ,  would she!  BF should be mad at his DD not you.  He can't be under the delusion you love his kid ?  He should be broken away from his ex.  Not taking nonsense phone calls from her.. actually at 18 there should be no phone calls between them.  Just texting   You actually have more problems then you think 

Own it. "I wrote what I wrote and I'd do it again!" 

That's on her, I don't feel bad for her at all. Reading someone's diary, going through their computer or a phone is always going to lead you to something you won't like. That's a good lesson for her. What next - she barges into your bedroom at night or on a Sunday morning, sees what she wishes she never had to see and goes around sharing her anger and disgust? She's an adult in YOUR house, she overstepped every boundary of decency. Any other adult who'd do that would no longer be invited. 
 

I say, keep your head high and remember you don't have to attend every argument you've been invited to. You can always answer any provocations with  a cheerful: "What do you expect to find in diaries other than people's uncensored emotions?" and change the topic or walk away to get on with your day. 
 

Hopefully all this means that she won't be visiting as much or at all.

"I wrote it in private in lieu of bringing it direclty to play with you and your toxic spawn at the time. Though now that your nosey rude ostensibly kidult daugther broke the seal, it is game on so be ready for the zero tolerance immediate ass baring of her toxic behavior and the failed parenting of you and your idiot XW."

Take a bow.

Madam,  Quality adults do not go into other peoples homes and read their private papers and then share that information with others and then to proceed to harass you about what was private information.  This private info not only includes diaries, but bills, wills, business dealings, phone messages, underware drawers, jewelry boxes, medicine cabinets or anything else that someone would normally assume to be private.  Quality individuals ignore that stuff or bring it to the owners attention that they left it out.  If any adult does this in your home, then normally that adult would never be invited back. You should not have to live in Fort Knox to protect your privacy.  What your SD did was beyond reasonable behavior and you and your DH should shut that nonsense down ASAP.  Rags response above seems perfect and could be directed at SD too.

Meaning that your stuff should be your private stuff. Someone finds your obviously private stuff, they put it in your room. Period.

Im assuming this is your home, of course.

If SD acts like a jerk, smells like a jerk, she must be a jerk. No one is refuting the truth of what you wrote, simply that you wrote it.

Your SO is out of line, but you have bigger issues from what you commented. Always "taking her side in everything under any and all circumstances" means that you are not a united team.