Having a friend is not a waste of time. If you were only friends with her in the hope of developing a love relationship, she probably feels deceived and used.

In my experience, if a guy friend says he's in love with me, it changes the dynamics of the friendship and that guy is no longer just a friend, but a potential suitor in a one sided love. It's uncomfortable to be around someone who is in love with me, when I don't feel the same way. The situation puts expectations out there which will not be fulfilled.

So, yes it "ruins" the friendship, but you probably should have caught on that she wasn't in love with you, when the physical aspect didn't really work out, and she continued dating others. In my opinion, there are guys you date, and guys you keep as friends. You have to be put in one category or the other.

Friendships are best when both parties have the same expectations of the relationship. She didn't waste your time - YOU wasted your time by wanting her romantic love, when she clearly didn't think of you that way. YOU didn't ruin the friendship, you simply each had different hopes or expectations for it.

I would suggest that you get on with your life, mourn the loss of the friendship, and be happy for the time you enjoyed together. Maybe keep the door open a crack, in case at a later date, she realizes that she actually does love you too, but let her open that door if she chooses. I'd say the timing might not have been right, but if you've known her for 12 years, something more would have happened than what you described. Let things settle a bit and see if she reaches out to you, if not keep yourself happy with other things.

If she's got a new guy, he's not going to like her having a friend who is in love with her too, that just can't work. Creates jealousy on both sides, you and him, and it's just not good. Often I think men profess their love for a girl when he thinks he's about to lose her.

Not really..if anything you should have made your courtship intentions known from the beginning.
Allowing yourself to drift into the "friend zone" was your mistake.

Still, just pick yourself up and ask someone else out. Plenty of women are looking for life companions.

You were locked in the "friend-zone" where she could use you for emotional support whilst giving nothing real in return.

She has done you a favor now by pushing you away. Push back and banish her from your life.

Let go and move on (yes, I know, easier said than done)