You don't need to overcome him, you need to overcome the memories you shared with him, just make yourself strong enough to live with his memories too. If you put in a lot of effort to get over that, trust me, you'll start thinking about it more and then you'll have one-sided expectations, and that hurts more than a pill. Just to overcome that.
You have three ways that I know--
1. Either take a one sided decision whether you want him in your life or not and start acting accordingly!!!
2. Try to move on (I know it's hardest to admit that you need to move on or that you need to forget him, but all the time, moving on doesn't mean you need to forget him) sometime it also means to live your life happily with your beautiful memories).
3. Just Think About Him So Much That There's Nothing Left To Think About (I Know This Sounds Crazy, But Only Think Of The Bad Memories You Shared With Him).
Still I understand the ups and downs you get, one day you feel you don't need him or you can live happily without him and then after few hours, you start thinking that you can live your life without him I can't even take the next step.
If you want him back in your life, make him feel your importance, make him regret saying no to the wrong guy, make him feel that there is no one better for him than you.
But all I would say is focus on your dream, believe me right now you don't know that you are not thinking about other things that matter in your life too but in future you will regret that something like In order to gain what was not yours, you lost something that really was yours.
Time heals all pain. Make her feel that you are a diamond, and that she is saying no to a diamond and that you won't even miss her once you find a new girl. Things you can do like go for new classes, make new friends, generate your new crush in the places you go daily, enjoy your life (this is a one time offer)
But all that matters in the final is that you take all your frustrations out in front of him, so that you can live the rest of your life happily.One thing that I apply in my life, can share with you.
"For every old pain medicine, there is a new pain (the old pain medicine is just a new pain)"
feel happy to answer this, good luck to you in future and don't worry i am also curing this pain i cure this pain by answering on quora or writing novel, you also do such activity Let's start with the thoughts that help you ease him (I'm not asking you to forget him, because there's no activity that can help you forget him). Just try to be a better version of you what you said yesterday, and one day you will move on and that day you will laugh at yourself about what happened to me at that time why was I so confused, why am I running after something was not mine).