I left my bf and we were TTC

First take a test. If you are give it a few days to calm down. Think about your options and what's best for you. If you chose to keep the baby don't let him force you into a relationship just because you're having his child. Don't let him force you into abortion either. If you chose to work it out I suggest both of you seek therapy and take it slow in the relationship. 

Tell him as soon as you'd had it confirmed by a doctor. I'm sorry he cheated. But no sense telling him in case you aren't and if you are, it's best to tell him right away to let him decide wether to be involved or not.

Sorry to hear about that. Try not to stress over it. Have you taken the test? I wish you all the best.