Oh theboob hugs to you,my ex left me with a 4 yr old and a 6 mth old for an 18 year old 4 years ago.
It hurts like hell,you imagine all what they're getting upto,that shes prettier,slimmer,etc etc , than you.When in fact ,they look foolish,trying to be something they're not= young again.
In time it will get easier,I know that doesn't help now,I begged and pleaded ,trying to get him back,eventually self -preservation set in.
I got myself together,made myself into the best person I could be, and started dating again.
4 years down the line I have the most wonderful partner , who is a dream with my dc,and has instilled my trust in men again.
My ex on the other hand,has just recently been dumped by his girlfriend(now 22), as he now can't keep up the pretence of being in his early 20's again (he's 32).
He is devastated,and was too upset to even tell our Dd ,when she asked whether they were going to see his girlfriend that weekend.
It may take time,but he will come to realise losing you and being a proper Dad to his Dc wasn't worth the fun he's having at the moment. By then you will be happier, settled and enjoying life and maybe ,love again.
And he will realise just what he's lost.