This is really a personal decision for you. The reason why you have decided not to have sex until marriage is relevant. If it's for religious reasons and you feel like you cannot reconcile your religion with having sex before marriage, then that is obviously something that could cause a problem for you personally. On the other hand, irrespective of where that belief comes from, you now find yourself in a relationship with someone who you love, who you have been going out with for an extended period of time, and who you would very much like to have sex with. I don't think there is anything wrong at all with reflecting on your own beliefs against that background and deciding to have sex with your girlfriend. In fact, I think that would be a positively healthy thing to do.
I will say that I think that deciding to save yourself until marriage is a flat out bad idea anyway. It is tied to outdated beliefs around virtue, purity and faithfulness. Not only is it not a bad thing to have sex with people other than the person that you marry, but it is often a good thing because it gives you insight into your own pleasure and experience of how pleasure works for other people. Far from being a sin, that is positive. Equally, sex is an important part of the vast majority of relationships, and sexual compatibility is something that you should really establish before you marry someone.
I am not saying that you cannot decide not to have sex before marriage. My view on such beliefs is just that; my view. But equally, as I say, I don't think it's wrong to reflect on any beliefs, and especially this one give the situation you now find yourself in. Sex is not bad or sinful, and having sex before marriage is not wrong. Sex is a lot of fun, and there really is nothing like having a good sex life with someone that you love. It's your decision, but there are plenty of good reasons why you might decide to have sex with your girlfriend here.