Should I leave her?

I know this isn't a dating forum but im gonna post this here anyway hoping some kind individual would help steer me in the right direction. And besides i've read worse on here

I met this girl at work back in January and we seemed to hit it off well. We liked talking to each other, hanging around with one another, had shit loads in common, and there was definately some chemistry between us. However I wasn't the only person she met at this job(of course). She met another boy, I call him a boy cause he's only 19 and shes 25. When they first met she told me she didnt want to have a relationship with this boy, and only wanted to be friends with him. This "boy" is one stinky bastard. He doesnt wear deodorant, overweight, going bald, and always pissed off at everything, among other things. 10 months later and i'm still single and she is now going out with this boy. Dont ask me how it happened but it did. I got nothing.

This boy is an asshole. As i said he's always pissed off, he has made fun of her on several occasions, he called her son gay(which really pissed her off), he has physically harmed her(by accident mind you) several times. Even though she said she didnt want a relationship with this "boy", she is now in a committed relationship with this boy.

Now this pisses me off for a number of reasons. The last female friend I had was 12 years ago and it lasted a month. I never hit it off with women. I have ZERO confidence with relationships because I havent had very many. Im an introvert by nature and im horribly shy so it takes me a while to open up to people. By the time I get comfortable enough with women they move on to someone else and it gets horribly lonely and depressing. I made the mistake of confessing to this girl that I did have feelings for her but apparently it went in one ear and out the other. She never said she had feelings for me, but she never said she didnt either. I still hang out with this girl, and she wants to hang out with me, quite regularly actually. She says she values our friendship and doesn't want me to stop hanging around with her. Doesn't want to stop being "friends". She flirts like crazy with me, uses sexual innuendo, even tries to poke me and touch me in various places. She says she gets bored all the time and wants company and yet when I go over to see her, he is always there.

This is where my problem lies. I have fallen in love with this women. Even before she started "going out" with this other thing. I have never had a love before so this is all new to me. Yes, im 27 and a vigin. The problem is I want a significant other so bad that whenever they are together and i happen to be there, i feel like shit. I feel like they are both mocking me, teasing the fuck out of me and its starting to get to me. It makes me feel about 1 cm tall. She knows I like her, she knows im a virgin, he knows i like her but he doesn't know im a virgin. So therefore since she knows this and it seems they are deliberately doing this in front of me, well lets say it gets horribly depressing. It makes me wonder why I even tried in the first place

I'm so confused, depressed, and alone right now. I want a significant other so bad its all i think about and to come that close, only to get the shaft really sucks ass. I dont want to stop, but I feel that sooner rather than later im going to POP and all this is going to come out anyway, regardless if he's present or not. I dont want to explode while he's there because it would not turn out well for him or me but if I dont figure something out soon, something is going to pop. 12 years of pent up frustration, anger, resentment, jealousy, etc is going to come out all at once

My questions are these. What should I do? I really do like this woman but I cant stand this guy. If she asks to hang out should I say no? I don't get to spend time with alot of women, especially one that seemed interested in me, so naturally im going to say yes. If I say no, should I explain why to her and see what she says, confess if you will? Should I just lie to her and make some excuse up? Should I just stop talking to her all together? If I did that then I would be alone and back to square one. Should I ask her why she keeps inviting me over to her place?

any advice would be kindly appreciated