Carmel dear - I am the nicest, sweetest person normally and never have a wrong word to say about anybody.
I know you're very young and I'm going to try not to be too harsh but I'm going to tell you something you don't want to hear.
If you think you're old enough to have this sort of relationship, you're old enough to hear the truth.
That last sentence of yours says it all -
"I'm not stupid" - yes, love, I'm afraid you are if you know he's cheating and you don't know what to do.
Let me say this only once -
THERE ARE OTHER BOYS OUT THERE
Now, wake up and smell the coffee.
And why are you bothering to post again? We're only going to tell you the same thing - to dump him - but you don't want to hear that.
And Norman is right - we don't have to post back to you but I for one wanted to try to get through to you. But I shan't be bothering again if I see a similar post.
Good luck and I hope you choose to do the right thing.