Make a list of all the reasons it won't work. Really focus on this, make it as comprehensive as you can. Avoid him when you can and go back over the list everyday.
Make it a thing to remind yourself of why you shouldn't be with him every time you think of him. Even if it's just one thing each time.
Also, make sure you're focusing on why it wouldn't work or shouldn't work rather than why it can't work. Can't introduces that forbidden fruit thing and makes it harder on you.
Failing all that, find something that symbolizes your feelings for him and use it to perform a banishing spell.