I assume you and your girl are under 20. That makes a big difference. There is a completely different approach for grown women.
The bad news is that this girl sees a big "L" on your forehead and reads it as "Loser". The other bad news is that you can't make a girl like you if you aren't her type. Your task in life is to instead find the girls that like guys like you, and then life becomes way more fun.
The good news is that it doesn't matter whether a girl has a bf, fiance, or husband, most of them will sleep with someone else, not necessarily you, in the right circumstance and timing. I've been with plenty of women who had bf's, fiance's, husbands, or vowed to be virgins until they married. It is not a limitation. Only timing and the kind of guys they go for are limitations. One of my bed buddies right now has a bf that doesn't know about us. She is a 21 year old sorority girl and I'm four years older than her mother.
To your specific challenge, if you have feelings for this girl, get rid of them. There are a million other girls out there, and you shouldn't be trying to pick one until you are past 30 (and even then, if you pick right, you don't have to settle for just one). If your intention is to f@$* with this guy, then ok. That sounds like fun. You need to realize that the girl will be crying in the end. Is that acceptable collateral damage? If it is not, then forget about it. You won't remember her in 20 years anyway.
So if you want to go forward, you must be uninterested, and by that, I mean UNINTERESTED, if she senses you are trying to make her jealous or that you like her, it won't work. You can't pretend, you really have to not care a s^#* about her. Second, instead of pursuing her, you need to have a plan to get someone SHE thinks is hotter than her in bed (not who YOU think). You want her to pursue you. Girls are attracted to the guy that other girls are attracted to, and the thought that you are sleeping with a hottie might get her interested in you, but ONLY if you are her type. If you aren't her type (there is no logic in women's preferences, don't even try to understand it), there is nothing you can do.
Once you have your hottie, the word needs to get to your girl that you are sleeping with her. When you see your girl, be the most charming person in the world, complement her shoes, but you don't make a move; she is nothing to you. You are uninterested. You are unobtainable. She can't have you. Wait until she is begging for you, (I repeat: It will never happen if you aren't her type). Then, when the time comes, make it look like you are doing her a favor, choose the right day of the month, figure out when she is ovulating and her bf isn't around, she'll be much easier, and the right event that she is interested in. Timing is everything. Be patient.
For your girl and your hottie decoy, as it turns out, all women are prostitutes; it is only a matter of price and terms. You can use this knowledge to your advantage.
You could try to go after this girl and the way I've outlined might work if you are her type, but if you want to be happy, don't. Look for the girl that likes guys like you. While you are focusing on the girl you like, you are not paying attention to the girls who are admiring you. Observe the other girls carefully. Chances are one or more have noticed you, but you haven't noticed them. Maybe have a round robin with your friends, "Who do you think likes me?" If you have female platonic friends, this line of questioning will work great, they will help you. There is someone out there wishing you would notice her. The clues are usually very subtle to the inexperienced. Figure out who that is. Ask THAT girl out. She will be glad you did, and so will you.