It helps to know that it isn’t forever. You wait until marriage, so there is a goal a mind. It’s hard when you’re with the person you want to marry, or will marry, but the point is that you’re not married yet.
And there’s the fact that it’s wrong. If one sees sex as a physical manifestation of their wedding vows, I am yours fully until death, you can’t promise that until married. Couples break up, engagements are called off. Yes, divorce does occur, but no good Catholic goes into a marriage seeing divorce as an option. And hopefully most people don’t go into a marriage saying the words ‘until death do us part,’ while wondering how far down the line they’ll be divorced. You lied with your body, is the basic idea.
Along with that sex has the dual meaning of procreation and unity-- most couples who have sex are using contraception, or are really hoping not to get pregnant. Which is taking out half of what sex is supposed to be.
Again, it’s hard. We are programed for certain things. It would be great if there was a switch that turned itself on when we got married, would save a lot of trouble, but it isn’t that easy.
What really helps is knowing you’re also helping the other person. Both are sinning if you have sex, some people don’t care about themselves enough to see it as a problem if they sin. But putting the person you are going to marry in a State of Mortal Sin… that’s a good curb.
Hope you find the answer you’re looking for.
God Bless!