How do I get back together with my ex-girlfriend?

First, you need to be real about what went wrong the first time around. If it was not something unforgiveable (abusive behavior, cheating) or unforgettable (you left for someone else) you might have some reasonable expectation of success. All you can do is to call, write and express your feelings for her and ask her to go out. If she says no, or is with someone else by now, then move on.