hi honey, i know how you feel. when i was 15 i lost my virginity to a guy in his 40s who also was not attractive. i was working my first summer job door to door canvassing and he invited me in for a beer, so i lied and told him i was 19 (legal drinking age in canada). i was not drunk (only two beers) and it was consensual but i felt uncomfortable and gross as it was happening, and afterwards i also felt gross and dirty, like you feel.
the reason i did it is because like many teenagers i was very insecure in many ways, i had awful self-esteem and it was the first time anyone expressed interest in sleeping with me. before this i'd only kissed. i felt flattered that someone "wanted me" and i thought maybe doing it with him would make me feel better about myself... but it just made me feel worse. afterwards i went home and cried and showered to try to feel clean again. i told my friend and instead of sympathy she thought i was gross for doing it with a middle aged unattractive guy.
i can think of something you can do to feel better - take action to protect other girls from experiencing the same thing. let this man know that ever since sleeping with him you have been feeling awful, gross, depressed, etc. you can write it in an email or a letter you leave in his mail box to make it easier. you can also say you never want to talk to him again and to please leave you alone.
this will take a lot of courage on your part, but it will be worth it because afterwards you will feel strong and empowered, instead of used and weak how you feel now. you will feel like a hero protecting others, instead of feeling like a victim or slut.
you don't have to take an accusatory tone or make him feel like he's a predator or an awful guy, because he probably was just ignorant to the fact that sleeping with you would make you feel this way. but be sure to tell him that he should never again do with other teenage girls what he did with you, because they might end up feeling the bad things you feel now.
he should have known better, that as an adult of his age that a teenage girl might be upset and ambivalent about a sexual experience with him. after reading your email/letter (or having a conversation with you if you are VERY brave), he will not have the excuse of ignorance anymore, and if he is a decent person, he will not sleep with teenage girls ever again, and even before sleeping with women who are significantly younger than him, he will take things slow and be cautious before jumping into bed.
he really should have known better already, but some people need some extra help figuring these things out. you can be that person who makes a difference.