Worried About Losing Someone Special

First off... I wouldn't say i'm broken hearted, but this seemed to be the best forum to post in. I'll give you some background on my problem and hopefully by doing so it will help me realize what I need to do to fix it.

I'm currently 25, i'm turning 26 very soon and I have not had a 'real' girlfriend in sometime. I've never had a problem with girls/woman liking me or finding me attractive, but for some reason I can never seem to get past that point and have a good relationship with them.
Within the last 6 to 12 months I have been extremely lonely and worried about where my life is going if I continue on this track. I believe I suffer from anxiety and or some type of depression. And I know its mainly because I miss having a female companion. I always feel great when I meet a new girl ( its like all my problems go away, so it makes me think that being lonely has led me to be depressed and its only temporary )

not to long ago, I started hanging around a girl that I thought was attractive and that I enjoyed being with. She was a friend of my sister's who's always had a crush on me, maybe its because she was my sisters friend or maybe because I was to shy, but I never acted upon it. Well lately we started hanging out and becoming close. We started realizing how much we had in common and I could sense that we both liked each other a lot. Now a slight problem is that she just broke up or took a break with her boyfriend of almost 2-3 years, so I have been hesitant on coming on to strong and scaring her away.
I'm so worried, because I had another situation like this about a year ago with another girl. She also had a huge crush on me but I believe I came on to strong and gave her the wrong idea that I only wanted a one night stand. What also happened is that when we tried having sex, I had a night that every guy has nightmares about ( I couldn't get it up ) I know this happens to everyone, but for some reason from that point on my confidence was just shot. Needless to say, she thought I was only interested in a fling, and she got back with her boyfriend.
It took me so long to get over that , and to be honest I don't think I fully am. Ever since that one incident a year ago, I think i've avoided intimate contact like that because I am worried that it will happen again.

So thats a bit of background on whats been going on... So back to whats been going on lately...

I have felt great and have been so happy since I have been spending time with this new girl. I've been going out more and not worrying about anything and really enjoying myself. I really feel like I am starting to overcome the problems that have plagued me for these last few years.
I was feeling like everything was coming together for me. I haven't even kissed this girl yet and now i'm worried that I might have waited to long to maybe make my move. The last 2 times now that i've talked to her, she's seemed a bit distant and not as interested as she was at first.
I am supposed to call her tonight and we will hopefully get together and hang out... But i'm so afraid that maybe she's worked things out with her boyfriend again and that i'll be stuck in this same rut again.
I don't even know if I have anything to worry about, for all I know, I could just be making this up in my head and just trying to find things to worry about.

Last night and today I have just been worrying myself sick that I let her slip through my fingers.
If she only knew how good she makes me feel inside. She doesn't know it, but I have felt like a whole new person since we starting hanging out.

So basically my fear is that she will lose interest in me and go back to her boyfriend. If that happened, i'm so worried that i'd fall back into a depression, just when I was starting to feel good again. I'm almost certain that would happen... Because look how much its been worrying me and I don't even know if thats the case yet.

So what should I do? She really understands me and we both have told each other a lot of things that we had never told anyone before, so I know the trust is there. Should I sit down with her and have a heart to heart with her? Or should I just keep trying my best to make her happy and not let her know how I feel ( in fear of her thinking it might scare her off )
and see how things go.

So please... I need some advice and fast. I do not want to let this girl get away because I feel like she's the best thing to happen to me in awhile. I really like this girl and fee llike we could have a good long relationship. I will be crushed if it doesn't work out :(