What was I lacking?

My long-term boyfriend broke up with me out of the blue and without clearly explaining why at the beginning of the year. As soon as we broke up he became a completely different person: he turned cold, almost evil, started going to clubs every weekend, even more than once a week (when we were together i always wanted to go, but he didn’t, so we just stayed home or did something chill with our friends), started going out to eat/do fun activities more (he always used the excuse of not having money - which was ok because he was unemployed, but he still does not have a job sooo), posting more on social media (while he NEVER posted me or anything else before) and cutting off all of our friends that he thought “took my side”.

About a month after the breakup i found out about a girl he was going out with, which he claimed was “just a friend”. Fast forward a couple of months, he is always posting about her and how much he loves her and it’s clear that they are dating now.
I’m just left wondering… was i really not deserving of this type of love? why did he do this to me? why was i not enough?

I know some of you will think “oh it’s probably just a rebound”, but he REALLY seems so in love with her. He is doing things for her that he never did for me - but why? what was i lacking?