Were you all having a sexual relationship outside of marriage? If you all were having sex, and she left you to have sex with someone else, then the both of you weren't doing what you should have been in the first place. The two of you were/are sinning.

The entire point of dating is to see if you are compatible to marry. Honestly, most people should know if they are compatible within a period of time much shorter than four years. There is never a reason to prolong a dating situation for that length of time. It's just wasting time. Sure, she should have broken things off if she wanted to see someone else, however, if there was no sex involved, then the two of you had no legal or sacramental ties.

There is nothing to be suicidal over here. Go to confession, focus on healing, and move on. Time and distance will give you perspective.