“Should I forgive him or let go once and for all?”

"Should I forgive him or let go once and for all?"
From a female blog reader:

Pls keep me anonymous. I have this guy i have been dating for some time now. He talks marriage and says he wants to marry me. We love each other and are like best friends. He showers me with love and all that. Everything was going on smoothly until recently. He told me his password is my name and i was like its a lie. So he gave me his instagram password which truly is my name and i logged in. For about two weeks, It didn’t occur to me to check his DM cos i didn’t want to be a nosy girlfriend. I wasn’t even logging again. Then last week I just decided to log in and check his DM and I saw his chat with his friend with him telling the guy “I want to to kpansh annie”. (hope everyone knows what kpansh means) Those were his words. Apparently his friend knows the Annie girl. Note i havnt seen him for about a month now cos he traveled to Abuja. And the Annie girl stays in Abuja. Now this is a guy that has been vowing to be faithful. I cut ties with him after that and have refused to talk to him after i told him what i saw. He has since been apologising, calling and texting and am not responding. And my kind of person i know i will just break up with him. My question is, am i over reacting? Or is this a sign that he’s a Fuck boy and i should just move on. Pls i need everyone’s opinions. He’s my 3rd boyfriend and i used to think that i over reacted in my previous 2 relationships before i met him. And am trying to just chill and stop walking out. But then again am scared that if i forgive him, he’s going to turn into a full blown cheat. What do i do?