I hear you. It sounds like you are in a situation that has become emotionally difficult for you. You're attached to a girl, who is a friend, who is also in a relationship with another friend.
This is causing thoughts and feeling such as, "I will never be as happy with another girl", like you have lost something forever and that can never be replaced by anything/anyone else.
It also sounds like since the beginning you have been in a place of resistance towards the painful feelings that you were having, rationalizing that they are obstacles to your life.
There is a well known saying, "what you resist, persists".
Therefore, along with becoming clear about what you want in a loving, intimate relationship - as holistic star was advising - I would encourage you to allow the painful feelings to come.
Not that you let them tear at your mind and heart, but give them a proper audience.
Just like a qualified judge will even give a murderer a fair trial to express himself, give these painful, hurt feelings an audience in that way.
You can be in the company of these hurtful feelings without identifying with them, and in giving them due attention, you may find that they begin to settle.