What if I have made a mistake previously, can I still have intimacy?
Another good question.
You can never go back, which is to say that you will always have to live with whatever consequences that might occur because of your past. That being said, God is not as concerned with your past as He is your future. Now that you have had a change of heart, will you follow through and stay pure. That is what grace is all about. You can be forgiven even if you still live with some of the consequences of your past.
Now, knowing that you can be forgiven is not an excuse to sin. Paul addresses this in Romans 6,
"should we go on sinning so that grace can increase? By no means we died to sin, how can we live in it any longer? Or don't you know that anyone who has been baptized into Christ Jesus were baptized into his death? We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father we too can live a new life."
We can be forgiven, and that means we have a new life. So commit that new life to living for Christ, out of respect for the sacrifice that Christ made for us. To use a military analogy, if someone in your unit made a sacrifice that saved your life in combat, you would always want to honor that person by living a life worthy of that sacrifice, right? It reminds me of the movie Saving Private Ryan at the end when the old man Private Ryan is at the cemetery and asks the question whether his life had been worthy. We will never be worthy of the sacrifice that Christ made, but we should live for him out of respect and gratitude.
The most important thing to remember is that God loves you and forgives you. You cannot go back and undo what has been done, but you can move forward from here and do the right thing. It will still be very special to your future spouse if you start from this day forward and save yourself for her/him, and I'm sure that you will be glad if they do the same for you.
On a practical side, here are a few things that you could do to make things a little easier to resist temptation.
1. decide ahead of time what your boundaries are. (a good way to help decide this is to imagine that you had a daughter and ask yourself what would be too far for a boy to go with her?)
2. never put yourself in a situation where it would be easy to go too far. (never at your place alone, never at home with no one around, never in the car somewhere... you get the point.)
3. don't do anything that makes you want to do everything. (draw the line way before you think you need to)