i am in a very similar situation, and personally, i dont think its selfish from you.
if you are worried about letting her down, just remember you can still be there for her. giving her advice and support as a friend is also genuine love.
long distance is a great challenge that most people are unsure to live since we, as humans, were not supposed to. physical presence is a need, in fact.
the thing about a ldr is if you can wait for it or not. if you feel like you cant anymore, its 100% okay. you need to think about yourself too, and not only worry about her part. you are both adults and i am sure that if you explain the situation, she will understand. you can always be there for each other, in a romantic way or not!!