ex left me for a 15 year old.

So me and my ex broke up about 3months ago he said he needed some time to himself. But he kept texting me and coming to see me telling me he misses me and I'm trying to figure out why don't we just get back together but recently I found out that he's been with this girl she's younger than us were 19 she's 15 so it's like I don't understand?? I treated him so good why would he leave me for a younger girl. He hasn't brought her to his house because his mom told me that she doesn't know who the girl is. So it's like wtf😭💀 any advice ? I'm trying so hard to get over him