Replaced after ONE WEEK

So a bit of background: my ex(who was my first bf) and I are still in each other’s lives because his sister is my bff and I occasionally babysit his nieces when nobody else can(Mom is a single mom).

I was babysitting last night, and their uncle-who my friend is also a caretaker for- started asking if the ex and I talk. And I said yes, because it would be hard to cut each other off completely since we’re both in his sisters life so much. And he said good, but he hopes I wasn’t being too friendly with him. Then he says how one day he got frustrated with my ex and he blew up on him. Then he says “you know, a week after he broke up with you, he was already running around with some other girl. And that really pissed me off. I wasn’t even supposed to tell you so don’t get me in trouble. But that made me mad because this I liked you”

I kind of laughed it off... but I fell apart as I put the girls to bed

A week. I can’t even wrap my mind around that. I’ve been spending almost 3 months upset and heartbroken, but apparently I didn’t mean a damn thing to him at all. All this talk of marriage and kids and everything... it was all a lie....And then to hide it... not like I need to know everything in his life, but he would feel the need to tell people not to tell me....

I guess this should shake me how replaceable and insignificant I really am... sorry for the rant.... I just can’t believe it.... idk what to do