Are the illnesses that afflict me after every sin a tribulation from God, and does God test those He loves? And how can I achieve sincere repentance and forget the past, and attain a good end while I fall into sins and then repent from them?

As long as you seek Allah's pleasure and fear an evil end, you are on the right path. Actualize this fear with righteous deeds, the most important of which is sincere and frequent repentance whenever you fall into sin, accompanied by remorse, abandonment [of the sin], and a firm resolve not to return to it. For Allah forgives the sin of His servant who repents whenever he sins. Illness is an expiation for sins and can be a sign of goodness if Allah intends good for His servant, as He hastens the punishment for him in this world. When a believer is afflicted with a tribulation in this world, he blames himself for what he has done, and this prompts him to repent and seek forgiveness. To forget the past, strive not to remember it, and occupy yourself with obedience to Allah in the present. If you do remember it, then regret what transpired from you.