I'm in my early twenties. I'm suffering from arthritis for several years. I took many treatments, but I'm not able to find a cure. I'm a Christian. One more thing is that I was in a relationship with a girl. I loved her like anything. I have touched her several times and I have kissed her, but I haven't been involved in sexual activities. Now she broke up with me by saying a silly reason that I don't understand.
My question is why am I suffering like this? I need a reason. Will God heal me? Will He forgive my sins? What do I have to do?
I can't answer why you specifically are suffering. I can only talk in general about why God allows suffering to exist in the world and those reasons are numerous. See: Why Do Good People Suffer?
I'm truly sorry about your arthritis; I know it can be quite painful and unrelenting. I have no idea if you will be healed in this life, but I do know that in the world to come we'll all be given new bodies and the pain and suffering found in this world will not be there.
"For we know that if our earthly house, this tent, is destroyed, we have a building from God, a house not made with hands, eternal in the heavens. For in this we groan, earnestly desiring to be clothed with our habitation which is from heaven, if indeed, having been clothed, we shall not be found naked. For we who are in this tent groan, being burdened, not because we want to be unclothed, but further clothed, that mortality may be swallowed up by life. Now He who has prepared us for this very thing is God, who also has given us the Spirit as a guarantee. So we are always confident, knowing that while we are at home in the body we are absent from the Lord. For we walk by faith, not by sight. We are confident, yes, well pleased rather to be absent from the body and to be present with the Lord. Therefore we make it our aim, whether present or absent, to be well pleasing to Him. For we must all appear before the judgment seat of Christ, that each one may receive the things done in the body, according to what he has done, whether good or bad" (II Corinthians 5:1-10).
But we can talk about your behavior. You haven't been acting properly as a Christian man when you sexually touched a woman to whom you are not married. "Now concerning the things of which you wrote to me: It is good for a man not to touch a woman" (I Corinthians 7:1). God is willing to forgive, but first, you have to give up your sinful way. "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (I John 1:9).
As far as why your girlfriend broke up with you, again, I can't answer since I have no idea what happened or what reason she gave for leaving.