How can i ever lose my bad name?

i have a really bad name for myself.. when i was younger i had with alot of guys, i am now 20 and still being called a tramp and not many girls like me.. lads always come up 2 me and try me and want 2 use me, i dont let them though, but when i was younger i was stupid and thought they liked me, even some of the lads said yesterday that i would go with them in a second and another lad said yeah i woulnd'nt be surprised, as if i was'nt even there.. its really beginning 2 hurt my feelings, i have a boyfreind and i think he thinks im a bit of a slag 2.. any idea how i can stop people thinkin this of me?? please help me. thank you