I've had several bad experiences of getting friend-zoned to the point where it's taken over my life. Thought it would be a good idea to offer some advice to those of you experiencing the same thing as it seems to happen all too often these days.
1. So you've become close friends with her and finally you've decided to ask her out.
The likelihood is that you liked her from the start but back then there were barriers e.g. she told you she didn't want a relationship (hint!), she already had a boyfriend, you couldn't gage any interest from her side (hint!) Now you ask her out and she rejected you but suggests you stay friends.
2. What do most guys do next?
All too often, guys feel hurt about the rejection but decide as they've invested so much emotional time in her that it's worth staying friends. Perhaps they feel that the opportunity may arise again to ask her out or the feelings will just turn themselves off. Sure it's possible to go back to being friends but there will always be that awkwardness or frustration over the rejection.
3. You start to see her around other guys/she gets a boyfriend.
This is when the jealousy comes out and your 'friendship' is put to the test. How will you cope if she's got another boyfriend and you're still into her? The answer is pretty badly; you'll get jealous, think he's not good enough for her and question why it is she always dates douche bags. This can lead to huge periods of angst and blame. You'll start to act differently toward the girl too.
4. Now is the time to move on.
Can you realistically be friends with someone you want more from? No. It puts you off dating other people cos you stay so invested in this one person who only ever wanted to be friends. You have to take responsibility for your own actions- look to point 1. If you can't ask a girl out who you know you like from the start, then it's not a good idea to just settle with friends.
My advice is to stop contacting the girl immediately as you need to look after your own interests. It may seem harsh but your friendship will always be one-sided. They say a friendship spirals downwards the moment one person develops feelings for the other. I myself fought that idea on numerous occasions and only after losing those people from my life was I able to finally get into a relationship. I didn't harbour any bad feelings towards the girls I cut off cos they meant something to me. It was just unfortunate that they didn't reciprocate and looking back, me trying to stay friends with them worked out badly on both sides.