No sin is bad enough for God to condemn you to eternal damnation provided that you're a Christian and have made a decision in your life to serve God instead of yourself. With that said, being a Christian does not give us permission to do whatever we want. We can't just aimlessly sin and honestly confess to God. That's not how it works. While Christians sin, we are expected to be more aware of what is right and what is wrong.
I think your problem is that you're treating sex as a purely physical thing, while sex also has emotional and even spiritual aspects that revolve around it, though sex can only be completely experienced in the context of marriage.
Sex in marriage is beautiful: not only does a couple have fun physically, but they also connect on a emotional level, with an overall feeling of togetherness, and become "one flesh," as the Bible puts itsex out of marriage has nothing spiritual to it, as it's a sin, and emotionally, feelings are fuzzy at best. While the sex may have felt good during that one moment, afterwards you may have second thoughts concerning whether doing this at this time was the right thing to do.
All I can tell you is this: from all I've heard, sex in marriage will be a lot less special on your wedding night if you have premarital sex, because it won't be something new. Sex in marriage is a bonding elementit brings a couple closer together. But outside of the context of marriage, it's the opposite: it could pull you and your boyfriend apart and completely ruin your relationship.