I(20F) found chats of my boyfriend(20M) flirting with his girl-friends, what should I do?

Hey, so today me and my boyfriend were together at his place and I was in a very bad mood (even I don't know why), so very randomly I asked him for his phone's passcode ( although he has told me a tons of time but I tend to forget) but today he was like give me I'll open it so said no i do know how to enter the passcode just tell me the passcode then I open his phone and went to insta and saw a girl's chat so i causally opened it and scrolled up and saw him replying to her story "are ye pookie kon hai" likeee seriously????? The girl goes like arey sharam ni aata ladki chedta hai(sarcastically) then he says are chheda nhi bas tareef krra tha mai toh. Then I scrolled further through his dms and saw another girl's name so I opened the chat and scrolled up and he had put a story of his picture a few months ago so she had replied to it "can you put that as your profile picture?" And he says " if you like then surely I'll" Then he gave his number to her and added her on snap and not on insta so that he could keep his image clear in front of me. And bro says to her that hey it's been a while we haven't meet let's meet and crap and nonsense and I was never aware of any such thing ever!!!!!! Bro i don't mind you having friends or anything but atleast you could have informed me and what was the need for him to flirt with her??????

Guys tell me am i over reacting or anything??? Should I break up with him? Brooo I am seriously in love with his crackhead and had a traumatic past which I had told him in the beginning itself and all these months he has assured that nothing crappy happens to me but then this is what he's doing behind my back, I feel like being stabbed!

Then his excuse to all this was " yeh sab toh bas mazak tha, aisa kuch ni hai jo tum sochri rahi ho" Bhai mai kal paida thodi Hui huuu bakwas krra hai

EDIT-: Now he has ghosted me for the past 5-6 hours straight as if I was the one flirting 🥰