Hello, I don’t know where to start but I need some advice from other step parents please (long story ahead)

A little about us, I came into my step sons life when he was 4 (I am “mum” at our house I do bedtime, bath time, dinner, school stuff, parent teacher interviews, sick days, play dates, school sport etc )at this time in his life his BM was seeing him once a week (she never bonded with him as a baby and had very bad PND) he lived full time with my husband, I absolutely adored my SS and we got on incredibly well from that start! (I have no kids of my own and am about 8 years younger then his BM) About 6 months into our relationship my SS’s BM decided she wanted to be a bigger part of his life (a bit of jealously with me around I think) we of course were so pleased because that is lovely for him. 

It is then that things got tricky we had 50/50 shared care, she is a bit of a “Disneyland Mum” no rules, no routine, eat whatever you want, no bedtime, unlimited screen time.. where as our house is always routine, structure, healthy eating etc. SS would come home from a weekend with her, exhausted, rude, grumpy and disrespectful because I supppse he got away with it all there.  Husband and I have had many a conversation with her about this but nothing has really changed. 

We only ever have good things to say about her in front of SS and would never stop him seeing her but now that he is 6 the time she shares with him without the rules and routine are starting to really effect his schooling! She has actually recently had a new baby and isn’t able to cope with both children so we have had SS full time with her seeing him every second weekend. And he is honestly like a COMPLETELY different child and thriving in school. 

I suppose for me I’m struggling now with how much time, effort, energy and love I give too a small person who isn’t “biologically” mine, I do more for him then his mum. But I can’t understand why at school we can have the most fun weekend and he can have so many awesome things to write about but instead will make up a story about something he did in the weekend with his BM and her partner which isn’t real... I don’t understand this it feels like a bit of a slap in the face, does he not want to be with us? Would he prefer to be with her? I’m trying so hard to be an amazing person in his life but I feel like all he wants is to be with his BM even though she isn’t good for him of course I know that is because she is his mum!

But how do you ever deal with the rejection and exhaustion from putting everything you have into another persons life knowing it will never be recepricated or appreciated?