I will keep this short. My girlfriend and I have been together for over a year. We have broken up four times, for maybe all together seven hours of being broken up. We are very in love with each other and plan on getting married. My question is if I have had sex with her already before marriage and asked for forgiveness but kept doing it still, when I get married will I be forgiven or am I a sinner for life?
A person is a sinner so long as he remains in sin. If you stop sinning, then you are no longer a sinner. This is true of all sins not just sexual sins.
There is two halves to repenting of sin. You have to change your attitude toward the sin and change your behavior. One without the other is not really repentance and I suspect that is why you constantly find yourself repeating your sin.
So, I have two questions for you. If you find it so hard to resist dropping your pants, why don't you just get married now? "But I say to the unmarried and to the widows: It is good for them if they remain even as I am; but if they cannot exercise self-control, let them marry. For it is better to marry than to burn with passion" (I Corinthians 7:8-9). It is far better to put things in the right than to continue to do wrong. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4).
Second, knowing your weakness in this area, why are you giving yourself opportunities to sin?