Well, I think God will forgive you any sin that you repent and will help you to overcome any temptation if you ask Him to do so. See 1 Corinthians 10:13: No temptation has seized you except what is common to man. And God is faithful; he will not let you be tempted beyond what you can bear. But when you are tempted, he will also provide a way out so that you can stand up under it.
So of course God will forgive you if you ask Him, each time you ask Him, assuming that you really do repent. But I don't know why that you feel that you can't stop - if you really believe this is a sin, a thing that means death, then your belief in God should include the belief that He will save you from giving in to the temptation itself, not just its consequences. As a Christian, you're a free human being not a machine or an animal - your faith enables you to do anything, morally speaking, that you really will to do, though it may be and feel very difficult to do it. If you mean that you really don't want to stop, and are finding it very difficult, I understand and sympathise but you are going to have move on from this way of thinking at some point; there is too much pain and cognitive dissonance involved in trying to live like that (that is, wanting to repent of a sin but hold on to it at the same time).
So you need to figure out if you really accept the teaching that this is a sin or whether, deep down, you believe the justification that you've being offering yourself - i.e. whether you believe that the fact that you love her and mean to marry her makes it good in God's eyes to have sex with her without actually marrying her. If you don't believe that, you need to decide what you will do with God's forgiveness once you have it - how can you prevent yourself from committing the same sin again? What needs to happen in your relationship, living arrangements, church-going etc? In any case, I think you need to make a choice one way or another. We can't pray St Augustine's prayer ("Grant me chastity and continence, but not yet") forever - it's not a very happy or stable mental place to be.