Catherine,
I'm going to be honest w/ you on several fronts here. Please bear w/ me. First of all, I think that Shannon M. made some very excellent points in her testimony of being married to an unbeliever. You should consider what she said very closely, as she is experienced in this area. Secondly, while the Catholic Church does not forbid dating/marrying non-Catholics, she strongly discourages it. Although you're only 18, you have to think ahead to a potential marriage. What would it be like after you had kids? What if they asked, "Why doesn't Daddy believe in God?" Would you be prepared to answer this question? Also, would you really be happy in a marriage to someone who you cannot share your faith life w/? Are you prepared to make such a sacrifice?
Now for more bluntness. Let's face the ugly truth here: society as a whole does not have much tolerance for what Catholicism truly teaches. Sadly, this also includes many Catholics. As a general rule (though exceptions do exist), a lot of non-Catholics have one heck of a time accepting our doctrines and teachings (especially when it comes to stuff like contraception and other sexual teachings- man is that ever a point of contention these days.

). You mentioned that your significant other is pressuring you to do sexual things w/ him- and it's making you unhappy. Catherine, this is a common occurance when one dates non-religious (lust is so important, for some reason

). I'm sorry, but it's true. This is the reason why I won't touch atheists/agnostics w/ a ten foot pole.
Now for a personal experience story. Last spring, I was considering entering into a relationship w/ a self-proclamed "devout Catholic." As I got to know him, however, I found that he wasn't so devout after all. He spoke in favour of contraception and population control. Also, he led me into sin by encouraging me to tell obscene jokes for fun, and tortured me w/ countless stories about watching porn, going to the strip club

, and tales of indecent acts w/ his ex. Needless to say, I did not get involved w/ this guy, despite sameness of religion.
If I were you Catherine, I'd get out of this relationship, and only date guys who are Catholics in the truest sense of the word. You deserve to be w/ someone who will help you get to Heaven, not someone who will drag you down to Hell w/ them. Yes, this type of guy is hard to find, but such individuals do exist. And, you'd be maximizing your chances of happiness in a relationship if you waited for such a partner (it's an exercise in patience, but well worth it). Isn't ultimate happiness the ultimate end here?
Just a thought. In the end, it's your decision, but this is what I recommend. I'm four years your senior and have never been romantically involved ever. However, I'm aiming for ultimate happiness, and if it's in His will, God will help me to find the right person.