What would you do?

I’ve been with my partner for 3 years, we have a 9 month old baby together, he has 3 children previously and I have 1.

I have just found out he cheated on me and was commenting on his 1st daughters mums instagram photos when we first got together, he doesn’t know that I know either.. now he has me blocked on instagram and Facebook and when I mention it he always says “I don’t go on it much” I am starting to think maybe that’s why he doesn’t want me on there because he knows he’s done the wrong thing.. it has honestly broken my heart into pieces, I’m just confused and don’t know what to do. 😞