Son, I'm going to have to tell you some things that you don't want to hear. But frankly, dealing with the situation now will save you heartache in the long-run.
"she never fucked anyother guy" - In all likelihood, this is bullshit. GUARANTEED this other "guy" is trying to get in her pants. Otherwise if he wasn't, SHE WOULDN'T BE INTERESTED IN HIM IN THE FIRST PLACE! Telling you this is just her trying to spare your feelings. You're both young. She wants to play the field. Her instincts are telling her to see what's out there. She doesn't feel the same about you anymore. But you lost your virginity to each other? She still CARES about you, but she doesn't want to BE with you. Do you see the difference?
There's an attachment there, a positive memory, possibly even a friendship. But LOVE? She's already fallen out of love. That's why she's put herself on the market. Why do I say that? YOU'RE 17! Neither of you has any **** clue what you feel. Almost everybody reaches a point in their life, when they look back on their "youth" and realize how stupid they were. This is especially true in the area of "love" and "relationships."
"you have no idea how much i feel like I need her in my life" - WHOA, big red flag. Need her in your life?
Do you love yourself? No seriously, do you love yourself? Do you understand your own needs, wants, desires, dreams, etc? Have you taken the time to inventory yourself as an individual? Probably not. And that's ok. Again, you're 17. Most people don't even START exploring introspection until they get out into the world on their own. But please, do try to consider it at least from this basic level: How can you give another person what he/she needs, unless you are secure in your own needs first? It's an important question, I hope you think about it.
Finally, I'll leave you with just a couple thoughts and a suggestion. There are a lot of women out there. A LOT. You already admit to looking (like the rest of us). Aren't you a bit curious what's out there? Don't you want someone who will not cheat on you? Aren't you worth that? Well, you aren't going to find something better unless you take the risk. Are you willing to be with someone who will likely stray from you someday? Will you settle for that? These are the questions you need to ask.
Also, I'm curious if you're the product of a single parent household. Raised by a single mother I'd wager. If you do have a father in your life, hopefully he is the kind of person you can go to with these questions. Mine isn't, so believe me I understand if yours isn't either. But preferably, every young man should ask his father about dating and relationships. That is very important. If not an option, I HIGHLY recommend you search the name Tom Leykis. I'll probably catch a lot of flak for mentioning him, but without a doubt his teachings have made me a better man. He does a live show. You can listen every day on your PC or smartphone. Used to be #1 afternoon drive show in the country, before CBS canned their talk radio stations. Now he's got his own business. Look, even if you don't ultimately buy into his pump 'em and dump 'em / use 'em and lose 'em strategy (and I don't either), you can still learn important things about signs, signals, red flags, and a lot about how men and women think. Or go read books on the subject, I dunno. Point is to get experience and knowledge from a male adult figure.