My fiancé is cheating

I am 20 years old, I have been in a committed relationship for over 3 years now, I have a 1 year old daughter and I am currently 30 weeks pregnant with my soon to be born son. During my relationship, we have had our ups and downs, he has cheated on me before, ignorantly I forgave him. He made it seem like he really changed and things were different but he's already back to his old ways and taking me for granted. He doesn't know that I know he's cheating on me on tinder. I have extremely bad anxiety, during my pregnancy I've already had a lot of anxiety attacks. Everyone tells me to stay strong but they don't realize how bad I am fucked up mentally. I honestly don't know what to do. I can't turn to family or friends about this, he leaves out of town at times for work and he is currently gone and won't be back until Saturday. . .Idk what's wrong with me or why I'm never enough? I've done everything for this man, I ignored what others said because I love him, he doesn't deserve me. I don't think I can marry him anymore.