I agree with some of the above, however, I think what you are asking for is not an answer as to whether or not you should LEAVE her? is that right?
You say you love her and that she is the woman you want to be with? Then be with her. You are 21 years old... you may not always feel that way about her - but equally, you may not always feel that way about another younger woman, a woman of similar age to you. You never know what will happen. Just be with her and accept her age if you want to be with her in the present moment. You are thinking way too far ahead in the future. There is ONLY the now, the present. I understand you dont want to let her down...you HAVENT let her down. If in the future this happens, then it happens - relationships will not always last forever. I am 39 and I dated two people who were 22 (16 years my younger) - not purposely, it just happened - we were attracted to each other. I was VERY aware of the age gap and it did feel a little odd, and people did comment and I begun to get confliciting 'advice' from people. BUT at the time I felt happy and as far as I could tell, they were happyI didnt think it would last forever, but for that time we made some lovely connections. Be happy, dont think to the future always and enjoy what you have xx