I am lost in a world i used to be so happy in!

It is going to sound so complicated probably pathetic to most of you who will read this. I am only 17 years old but please don't think i am some loved up teenage girl who just wants to be loved. If you can understand what i mean by that. I have a boyfriend, i have been with him for about 8 months now. We are really happy, get on really well as a couple and as friends. Its just recently my head has been driving me crazy. In the summer my boyfriend went away with his friends to a plce called Newquay for a week for a holiday. You can imagine down there they lived a life of alcohol and clubbing. One night my boyfriend got extremely drunk and ended up kissing this girl. I found out the night he came home. I was devastaed. I had thought maybe something could happen from me being away for two weeks before hand on holiday and then he went away. plus being a 17 year old boy and very drunk i could imagine something may happen. Everyone told me that he wasn't like that and that they could never imagine him doing something like that which made it worse when i found out. We overcame it though and we stayed together. We find it easy to talk about what happened and i did find out the first night he was home. We got back to normal and everything was fine. However recently i haven't been very happen within myself. I find it easy to cry most of the time, i hate being by myself, i am always thinking about him, i am stressed with a variety of mood swings. I feel depressed. I knew it would be hard to trust him again and i thought it would come with time. However recently i have felt so paranoid and stressed out over him. I can't relax. I feel my head is so messed up and i watch him when we are out convince myself he isn't being himself around me, most of the time when we go out i tel myself its just because his friends re there and i know they put alot of presure on him, you see most of them are single and i feel that they may possibly tease him. I just can't talk to him about it. I don't know if i am so paranoid because i haven't learnt to trust him yet or what. I am so confused. My friends think i am paranoid but i really feel that something has changed in him. I love him alot and i know he loves me but maybe he just needs some space. I am so confused i really think i might have some sort of emotional dperession or paranoia. Can anyone help me? I am getting worse and worse!