When I first started my walk with Christ I dealt with this problem. I couldn't come to terms with my past.
Although I pretty well have conquered that now, I do tend to notice here and there that something from my past or fleshly nature will pop up in my mind. Something that I know is sinful, yet it creeps its ugly head in every now and then to tempt me, whispering something like: "Come on, you used to enjoy this, you still could." These kinds of temptations creep up in a lot of people's minds, however, I choose to fight those temptations since I have chosen to follow Christ and His Word.
Some would call that Satan trying to steer you from the path of righteousness. I personally don't believe that it's Satan standing there whispering in your ear, but I do believe that it's the human's sinful nature; their flesh. My father-in-law calls it 'flesh-talk'.
In situations like these I find that prayer really helps to combat the sinful nature of 'flesh-talk'.
Should we forgive our sins of the past? Yes, I think we should forgive our sinful nature of the past. Since we have been cleansed by the blood of Christ we have been reborn and given a second chance to make the right decisions. However, I believe that it is important that we look back on our past mistakes as a reminder to grow and steer away from them. Learn from them, but don't repeat them. Those sins are in our past, so we needn't allow them to affect our present and future.
So,
"Then shall he also say unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels." - Matthew 25:41
Let us always remember that no matter what Satan or the flesh throws at us, we who believe in the Lord Jesus shall prevail. Just read the Bible. The ending is not pretty for the devil and his followers. There is only one outcome for them.