It's always so tough when there are joint kiddies involved, if not for your son, I'd say that if you're thinking about leaving then you should. Are you concerned about paying the rent alone - or are you worried that your GF can't afford it!?

It's not clear why you are thinking of pulling the plug.  For instance, is it just that you and her are not compatible?  Or, have you determined that she has character deficiencies?  If it's character deficiencies, will your child be at risk without you around?  These are tough questions that you might want to consider working through with a therapist by yourself.  Wishing you the best.

First, if you really do want to leave, consult with an attorney to present her with a fair parenting plan to be signed and submitted with the court before either of you packs a thing. 

2nd, I wouldn't wait for another 11 months if I was unhappy. Read your lease and see what it would cost to get out. You might only have to give 60 days notice and pay a fee- which could be worth it. OR your GF might be ok with living there and getting a roommate or you can do the same. 

If you split you have to assume that one day there will be step parents involved in your kids lives, all we can do is hope that our exes choose well. Keep communication with the kids open and hope that they aren't alienated and feel free to talk open and honestly. 

Make a plan to get 50/50 custody of your son, since you aren't married this isn't automatic so you will have to file. 

You don't have to settle for someone who settled for you!

You found out that she was considering going back to the first baby-daddy, through texts between them. That was a huge red flag - emotional infidelity is as bad as physical and who is to say that there was no physical. And now you are not happy. Im sorry things have come to this.

1. If you are still physically intimate, either stop or start using birth control. Also, get tested for STI/STD.

2. Consult with local family law attorneys about parenting plans and child support, find out what your options are. Sometimes the unknown is scarier than the known.

3. Sure it is hard to know that you will be separated from your little for half their life, but if you are this unhappy then it will translate to kiddo. You can TRY staying together for the sake of your child, but living together in name only is very soul-sucking.

Keep us posted - post as much as you need to!

Bottom line is that you don't trust her (according to your other post). Being in a relationship with someone you don't trust is a miserable way to live.