They don't say "strike while the iron is hot" for nothing.
Women respond well to a guy who does what he wants, when he wants it regardless of the chances of rejection or humiliation. They salivate and wet themselves for such a man. By contrast the guy who plays it safe, subconsciously looking for their permission or a "sign", is about as appealing as leftovers from last week.
It's a little more involved than that obviously. YOu do need to get some kind of read on the female and the situation before you act. With experience you learn to read between the lines and decipher the subtle cues that women drop (90% are non-verbal). There is no magic formula for this I'm afraid. Only by trial and error will you develop the necessary instincts and intuition to guide you.
Over time you will see that women give us certain windows of opportunity, and if we dont slip in there like a thief in the night, they just assume we are not interested, spineless, gay or all of the above.
In this particular situation you let the iron get cold (we are talking 6 months of freezing!). Componding the matter she already has a dude in the picture. Your uphill battle has become a cliff-scaling nightmare which will not only waste a great deal of your time, it will leave you in a deeper hole once you lose your grasp and fall.
Now if you really had your act together, you could probably recover and get her on your team (provided this new guy wasn't really doing it for her). If you had a nice rotation of lady friends and weren't hung up on this one girl, you MIGHT stand a chance to pique her interest. But the very nature of your question illustrates that this is not the case.
This may be hard for you to hear, but you never had her and probably never will.
Why?
Because you lack the balls to take the necessary risks. You also lack the emotional intelligence/experience to really connect with someone who is "yours" only in your fantasies.
If it was me in your shoes, I'd chalk this one up to a learning experience and move on. Work on getting over your fear of rejection - learn to value yourself INDEPENDENTLY of what any woman thinks, and you will be leaps and bounds ahead of most guys in the game.
That value has to be REAL though. Saying "I'm the sh*t" means nothing if you haven't put your work in. Practice, practice then practice some more. If you are really nervous around women, get over it by befriending a lot of them without any expectations. Emerse yourself in beauty till it no longer phases you, or impairs your ability to act.
Remember, trying to get any one girl is a statistically proven failure. You have to have a lot going on in your life, and several prospects orbiting in order to really hook anyone worth your while.