I say do what your heart tells you. This means don't rely so much on what your head tells you. God did say not to lean on our own understanding, but to trust Him instead. If you really feel strongly in your heart that you should propose, then I think you should. I normally would advise against it with someone your age because you're a bit young but you seem mature for your age. Just be prepared for her to say no. She might want to wait. Whatever happens, know that its God's will and don't be discouraged because God always wants whats best for us.