You didn't make a mistake because she was also itching to get out of the relationship. She's going to date others and may well find another love. You will only get a chance if she's still single - and even then she's making no promises. And as it was, the two of you can't compromise so this will still be an issue down the line.
It's not at all unusual to doubt ourselves after ending a relationship. My advice is to block contact with her - including unfriending her on FB. Explain to her that seeing her online will make it harder for you to manage when she's not your girlfriend. If she gets offended about you unfriending her, simply tell her that if she remains your girlfriend you will have no need to unfriend her. However, if doesn't want a relationship with you then she must do without you in her life.
Yes, it sounds harsh - but no contact is the way to go. Either 1. she will miss you and want you back or 2. you will both get over each other more easily. But she's certainly not going to miss you and appreciate you if you remain part of her life.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.