I'm a Christian. I have been having this challenge of sex between me and my girlfriend. Our relationship was sinful to be honest. We both felt guilty and decided to repent of our fornication. Sometimes I find myself wanting to do what I preached to her against. Afterward I always repent again. Please, I need your advice on how to go about this stuff. It's really weighing me down.
As much answers as I get, may God bless you with wisdom in Jesus' name, amen.
It sounds to me that several things are happening that is causing your desire to no sin to fail.
Repentance is a change in mind that what you were doing is wrong. You no longer excuse what you do, but always seek not to fall back into the sin, even if it is an inconvenience. Repentance is also a change in behavior. Without both a change in mind and a change in behavior, you will fall back into sin. "For godly sorrow produces repentance leading to salvation, not to be regretted; but the sorrow of the world produces death. For observe this very thing, that you sorrowed in a godly manner: What diligence it produced in you, what clearing of yourselves, what indignation, what fear, what vehement desire, what zeal, what vindication! In all things you proved yourselves to be clear in this matter" (II Corinthians 7:10-11).
Where I'm guessing you are failing is you are focused on not having intercourse, but you are allowing yourself to get involved sexual behavior that leads up to intercourse. Sometimes you are able to stop before things go too far, but more often instinct takes over and you get caught up in the moment. It isn't until after you ejaculate that you realize you sinned again. You may have even experienced times where you are thinking I shouldn't be doing this, but it feels like you are disconnected from your body and you can't stop yourself from taking off your clothes and entering her.
What you have to realize is that you can't play with fire and be surprised that you sometimes end up being burned. See: How much sexual stuff is too much?
Another possible problem is that you are not allowing yourself to ejaculate often enough. You got into a habit of using sex as a release, but in order to stop you have to have another way to relieve yourself of the semen you are producing.
Finally, if your girlfriend is not as committed as you are to stopping the sex, then you are always going to face temptations. Some girls get the mistaken notion that the only way to prove that a boy is interested in them is if he is having sex with them. Thus, when a boy says he wants to stop having sex, the girl hears this as the boy is losing interest in her.
Stopping fornication is only half of the problem. Once sex enters into a relationship, it quickly dominates it. Most of your time together revolves around sexually simulating each other, by words, by visuals, and by touch. The result is that you stop getting to know each other as persons. Thus, to counter what has happened, you need to fill your time together with non-sexual activities and conversations.