Are you RC, OP? They have an awful lot to say about sexual morality, much of which is well-thought-out lifestyle advice.
Artificial contraception is out because it invariably leads to an imbalance of power within a relationship (one person or other has to take responsibility for it-think about the pill, the condom, etc).
Sex outside of marriage is out because sex quite often leads to babies and it is self-evidently easier to raise a child in a loving, committed relationship than having to do it by yourself/in an abusive relationship/insert your own difficulty here
Marriage is a one-time-only offer. Divorce is out, not because the Church doesn't recognise that some marriages are not meant to last, but that when you commit to the very specific vows you take in a church, it cheapens it if you don't enforce it. Annulment is there for situations where it should never have been. It's not meant to be easy or populist.
I could go on for pages here but I'll spare you .
If you don't like it, no-one is forcing you into it, and you can leave at any time.
OP, these rules are all well and good, but they are designed around an ideal set of circumstances that real life often prevents us from having. You do the best you have with what you've got. But I think the most important doctrine preached by Christianity in all its forms is the concept of absolution. No-one in the Christian tradition wants you to feel guilty, not your God or your priest or your congregation. You go to confession, get it off your chest, do your penance and move on with a lighter heart.
Discuss with your DP (I really think you should: religious beliefs can be as final a deal-breaker as sex). Explain how you feel. Try abstinence and see if it works. And don't beat yourself up if you find out you can't stick to it.