At your age, there is no crime in taking things slow before deciding to commit. You’ve been upfront about your intentions all along and have let your relationship grow and evolve to the point that you are ready for the next step. I wish more young people would do this. It would prevent a whole lot of pain that comes from committing too soon and then having to put both people through a breakup.
Unlike you, your girlfriend hasn’t been totally honest. There was no need for her to lie – much less lie numerous times. If she is being emotionally blackmailed into staying with the other guy because he is threatening her, she needs to find a counselor to help her learn how to protect herself while she breaks things off. In that case, she needs your support.
If, on the other hand, she is using that story as an excuse for holding onto the other guy while continuing to date you, you have another issue. Why would you want to give your heart to someone who lies to you and can’t fully return the love you are prepared to give? If that is the case, it is time for you to let go and find the kind of woman who will appreciate your caring approach to relationships.
You are only stuck in a “love triangle” if you stay in your corner. As hard as it may be to do, I think it’s time you took a break to see what happens.
I wish you well.