Struggling with Infidelity and Isolation as a Stay-at-Home Mom

my old man of 5yrs has cheated alot in the past and been caught I firmly believe he's cheating again with multiple people and I've gave up caring and has led to me being bipolar with undiagnosed anxiety depression and paranoia. my heart is so broken that it can't break anymore. we have a soon to be 4yr old daughter and I'm forbidden to leave or his mom will fight me for custody. I'm a stay at home mom mainly by choice. her daddy is rarely home and she's more content with me then anyone else besides her dads mom. I put on a happy front as much as I can around her while my heart aches and breaks. he's always said he likes bigger girls and I gained over 100 pounds since I've had my daughter leaving me around 375+. I love him thou I cry when we fight and I swear I'm done with him when I know deep down I'm not. yes all this is on my mind at this moment why? well because he is never home he's staying with a girl he met from a job he got fired from. a girl that has sent him nudes and has voiced she wants him. 3 girls from his old job want him and 4 if u count one of the girls sister in law. yes one of the girls is married. he says I'm just no fun anymore. I'm about to be 32 and I'm reduced to being nothing but a mom. I don't go out, me and him never do anything together I'm literally a stay at home mom. no fun no friends no nothing. I'm alone to the point I honestly believe the only one there for me is my 4yr old daughter that believes her daddy is always at work. I'm drained... emotionally, physically and mentally.