It's due to you having a low self-esteem and it can be really hard to regain it again, probably years when its been lost. Bullying is a terrible thing, I know how it feels. I was bullied for being quiet in secondary school until I was 15. The bullying just made me even quieter and made me loose confidence. I've gained most of it now, but I must admit I am still a quiet person.
As for the sexual problems, try not to think about how these bullies have made you feel. Think about your boyfriend and how he loves you. He means what he says when he says your sexy, he loves you, etc. He wants to make you feel special, believe him. Focus on how much he cares about you when having sex. If 'dryness' becomes a problem, lube might help. Hope this helps