It sounds like you’ve already made your decision but want to double-check your own thinking. I’m reluctant to make a judgment because I don’t have enough information about the whole of your relationship. Yes, your husband lies and cheats. But you didn’t say if there is anything positive about this marriage or if he is doing anything to take charge of himself to be a better man.
I do know two things: First: You can’t make a marriage on your own. It takes two people who are willing to be worthy of each other’s love and trust. If you have to constantly monitor him, you don’t have a partner, you have a fellow who doesn’t want to grow up and be a responsible husband and father. Nothing you can do will change that unless he sincerely wants to change himself. Second: You two shouldn’t even be thinking about another child until you have a solid marriage. Another child won’t bring you closer. It will put more stress on an already stressed relationship. Another child won’t give you the love you need from an adult partner. Children need us to love them without expectations that they fill a hole left by a grownup.
I’m very, very sorry that at only 20 you are having to face even the possibility of a divorce. I’m sure this situation is disappointing and heartbreaking and frightening. If you need someone to help you think things through, I hope you will seek out a counselor to help you feel good about whatever decision you make and to help you figure out how to take care of yourself and your little daughter. You are fortunate to live in a city where there are many qualified therapists. Ask your pediatrician for a referral. If you have the kind of family that can be there for you during this difficult time, I hope you will let them love you and help you. You deserve to have some support and practical advice.
I wish you well.