break up advice?

So my boyfriend of 3 years and I just broke up a week ago and it was unexpected but expected, we'll not even a week later on New Year's eve he posts a pic of him kissing some girl on snap chat, but I had him blocked on knsta and snap so I found out from others. So we're really done. I've been talking to other guys just ask friends because I'm for sure not ready to date yet, but why does it still feel wrong when things get even semi flirty? Im 19 he was my first bf so this is my first break up and this is all new to me. Any advice