why is it so difficult for a teenage female to reach orgasm?

You would be very lucky to achieve an orgasm through sex. You need a man who knows how to make that happen for you, which is quite unlikely if you choose a boy your own age. Most boys your age do not have the experience needed nor do they have the desire to please you. Generally they are interested in their own satisfaction and bragging rights.

If you are lucky enough to have a boy who really wants to please you, you will have to assist him by showing him what does it for you. Every girl is different as to just what works for them. This is what you should have learned from years of masturbating. If you haven't learned that yet, you need to get started ---- now.