The most important thing I can tell you is that God knows all about your future, and nothing will ever happen to you that catches Him by surprise or is outside His love and concern.
In other words, just as God was with you during your past struggles with cancer, so He will be with you in the future - no matter what happens. This is why the most important thing you can do is to learn to trust Him for the future. The Bible says, "Those who trust in the Lord are like Mount Zion, which cannot be shaken but endures forever" (Psalm 125:1). Jesus said, "Do not let your hearts be troubled. Trust in God; trust also in me" (John 14:1).
If you have never done so, ask Christ to come into your heart and life today, and trust Him alone for your eternal salvation. Then ask Him to help you turn to Him every day and trust it into His hands. If you can trust Christ for your eternal salvation, can't you also trust Him to be with you every day? Of course.
Does this mean your cancer will never return? I wish I could promise that - but of course, I can't. But I can promise this: No matter what happens to us, when we know Christ, we know He is always with us - and ahead of us is heaven. Put your trust in Him, and your fears will begin to fade.