If you've spent years in sin, repent. If you feel like your sins outweigh every good deed you've ever done, repent. To despair in the mercy of Allah is a trick of Shaytan. And as you near your death, Shaytan will try even harder to create feelings of fear and despair, but as the Prophet (s) told us in his final illness: "Let not one of you die except with a good opinion of Allah."