There's no guarantees that you can , of course - but how long have you been apart, and how and why did you break up? Whose choice was it? Have either of you moved on to other loves since then? Does she have a new sweetheart?

If she has someone new in her life, it's best for you to step out of the picture gracefully. Let her know - if it is true - that you are sorry that things ended between the two of you, and that you would like to see her again in the future if things change. Be a true friend to her, if she's open to that. But don't lurk, and don't hope for or try to engineer a break-up with her current sweetheart. That isn't the way a true friend would behave, and it isn't loving.

If she hasn't moved on, do the same, and be willing to be her friend if she is open to it. Can you love her as a friend if that's all she's ready for? Do you love her that much? Be available and supportive, and prove that to her. 

If she will not allow you to do that, move on. It's her loss at that point. I know that is hard, but doing otherwise will just cause bitterness.

Good luck to you.