People who are in love with their lover don’t spend 90% of their time thinking about someone else. You are only 20. This is the time of life when it is normal and appropriate to explore a number of relationships before settling down into marriage.
You are being unfair to your girlfriend and yourself if you stay in this relationship. Blocking messages clearly didn’t block your ex from your life. I don’t know if your ex is the one for you. Neither do you. But I do know that it’s only a matter of time for your current relationship to fall apart.
Yes, breaking up hurts. Your girlfriend will be sad and angry. But she will be sadder and angrier if you continue to pretend that she is your one and only when you already know that she isn’t. I hope the two of you can part in a caring way and wish each other the best.
I wish you well.