You sound like a nice guy... but from a guy who has been there done that, let me explain a few tihngs (from my expierence).
Fist, you can't change a girls mind. Remember this, it will save you a lot of heart ache in the future.
Secondly, If a girl says that she wants to try something else, but continues to talk to you? it means that she doesn't know what she wants. SEE THE FIRST RULE.
Lastly, If a girl says that she wants to be FRIENDS? it means just that, but also that she thinks she is doing you a favour by allowing you time to get use to the break up.
I don't mean to sounds sinical(SP), and as much as this will hurt...
I really think you need to break ties with her. Maybe not entirely, but just start going out on your own and doing things with other people. You don't mention your circle of friends, so try doing things anyways. Even if you have to do it alone. You are 20... So you are either a college student, or working full time or both. IF you aren't in college? GO! IF you are in college, then get involved in an organization. You will meet new people and learn that love is like working out in the gym. The more you do it, the more you push your limits, the more you can do later. Sounds corny, I know, but its the expierence that matters here.
It always hurts when someone we love decides that they don't want to be with us anymore. Espically when the relationship is percieved to be going so well. I think it is great that you are reaching out for answers. Give this girl some space. IF you don't want to give her the wrong idea, and you DO want to leave the door open for her to come back, then don't do anything that would piss you off if you were in her shoes. Either way, get out of the house, enjoy life, take up a new hobby, and if you are not doing anything with your life, meaning you don't have a plan for your future laid out, then I highly sugest you make one. Start small, say with a 5 year plan. Something like go to college, working out more, getting a place of your own... small things that are easily achieved, and if you achieve them all before the 5 years is up, AWESOME. Make a new plan and keep going. Next time, while you are making a 5 year plan, start to think of maybe a 10 year plan as well.
Just some ideas from a man who has been in your shoes...
I hope it helps,