So you are aware that he regards you as nothing. He got a girlfriend and ditched you; this is quite common, I’m afraid. People always switch up and get blinded by love once another person is introduced, this is why people normally end up with no friends after the break up.
You are just jealous that he has got a girlfriend, you missed your chance and there’s nothing that you can do. Given the fact that he mocks you; I think moving on will be your best bet. Probably use this as an experience and appreciate that if your feelings for someone are there, or if you think it’s there, always remind yourself that this person on the receiving end might not feel the same way about you. It’s best to show your affection towards the person early on and see how they react to it; if they completely ignore it then you clearly know that they don’t like you. This should give you closure.
You are now in a position where you didn’t quite express your feelings while you had the chance - which is ok, just learn from it. You probably need to find another person to talk to.. before you do, make sure you completely forget about this guy though as you constantly thinking about him will affect your new relationship. So in the meantime - focus on your emotional wellbeing, find a passion & focus on your future. The right person will come when you’re ready, there’s more to life than love.