zachtos,
I think you know what to do. Marrying her would be your biggest mistake you've ever made in your life. She's not the girl for you. No one should be "obligated" to stay with anyone. It's about your happiness, not her's. If she threatens to kill herself, that's her problem, not yours. After I read that you two were dating since 16 years of age, I knew from the start that this relationship was headed straight for the wall. I honestly believe that people should experience other people (i.e. date around) until they know exactly what type of person they want. At 16, you will never know what type of person you'll want because the vast majority of the 16 year olds are clueless and haven't seriously thought about life after marriage. Leaving now will be much easier than leaving after marriage. Imagine if you have marriage, kids, and joint properties in the equation. You'll lose the kids, you'll end up paying child support, alimony, and you'll most likely forfeit half of your assets to her unless you got her to sign a pre-nup.
By the way, there is NOTHING wrong with being single. I don't get why people are "afraid" to be alone. Don't be in a relationship for the sake of being in a relationship because it's not healthy. Once you break up, you'll feel sad but it doesn't take long to get used to the bachelor life. Do you know how men get over past relationships? They go out, party, and nail any girl who'll say "Yes".