Your not stupid darling, that is exactly how i felt when i was your age and still now.
I can tell you this though your not in love with him nor do you want to marry him if you want to go out and try new things. If you loved him you'd be be content.
My advice is to really think about how you feel etc. if you don't want to stay and wish to go out and be single and playing the field (which is what 18 year old's aremeantt to be doing) then break up with him before you do something like cheat.
You also don't want to be miss leading by carrying on the relationship as it is unfair on himas welll if it not going anywhere.
Personaly you never get back these days and id spend them living it up as young and singe!