We have been in a relationship since childhod. We grew up together and we both are Christians. We are now in our early twenties.
We have been kissing, fondling and touching our private parts for the past few years. But we have kept it very strongly in our minds that we will never do sex before marriage. We have been practicing this for years now, so it is certain that we won't have sex before marriage.
We decided not to give a hand job or do fingering. As we need God to help us in our relationship, so even God expects something from us. But every time we meet, we still have some physical desires, which means lust, which is also a sin according to the Bible.
For me she is not my girlfriend, she is almost my wife and I am like a husband to her. We are very sure we won't marry anyone else except each other. So it is quite obvious, as we are humans, we have desires about each other.
So how do we avoid it?
"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 isn't that you two would not end up having intercourse. It is just that you've managed not to do so so far. You cannot count on the fact that you've resisted the temptation to go further in the past as a guarantee that you would be able to do so in the future.
But as you noted, it isn't that you've been avoiding sin. You have been sinning all along through lust and lewd behavior.
Not long ago I gave a list of rules for reducing sexual temptation before marriage. See: Can you give some guidelines for how to behave in courtship? Even though you have gone further in the past, you should follow these guidelines for the future.