My opinion: There is no point going for someone that wouldn't love you whole-heartedly. If she does, she would have left him and went for you. Or she is trying to make you do something for her, maybe to use you to break-up with her current boyfriend.
Reasons as follow:
1. She has a boyfriend.
Despite what the unknown reasons as to why she haven't left her boyfriend, she is still together with her boyfriend. Even though she said she likes you and would have gone with you if she was single, we girls often use that line to say it as a compliment to a nice guy who we are not going with at the time, kind of leading you on in a sense. If we have really liked you, we would want yo make you our only one. So there is a high chance that we just in the process of deciding on our options before us. I am just using a generic term, not everyone is like so. Nonetheless she is taken, and it is stupid to be putting yourself up for a drama for you, the girl and the boyfriend.
2. You guys have kissed
It is needless to say, but kissing is a sign of affection. But sometimes, kissing with someone does not mean they necessarily love you. It might be that she wanted to try something new after having the same old thing all this time. There could be so many reasons as to why she have kissed you e.g. under the influence of alcohol etc. But in the end, a kiss is not a make/ break factor. A few kisses does not hold a proportionate value to your decision as whether to take it further or not, in comparison with the context. Also, kissing her more does not help the situation any better than it is at the moment, it would only add to confusion and perhaps triggers rumour, making it worse. So you need to figure out your feelings, do you like her (to what extent), how far are you willing to go to get her and is there a foreseeable future between you and her (mutually)?
3. She is spending most of her free time with you.
You need to figure out if she was trying to be flirtatious with you or were you guys just hanging out as friends all this time. This is because as a female, I enjoy hanging out with guys just as much as girls and sometimes, I solely want to hang out with my guy friends because it makes me feel like I have greater male connections other than my boyfriend or whoever I was being intimate with at the time. Of course, me and my guy friends never did anything other than talk about games or matches, but we do spend quite some time together (and my 'boyfriend' is informed of this). So you need to figure out that during the time she spent with you, is she being flirtatious or just for pure friendship.
In the end, you need to evaluate for yourself if this girl is really the one for you and if you got what it takes to take the situation further.
May all the wishes be in your favour.