The question is not whether or not God can heal you, but will He. Of course we know God is capable of healing any disease for He made up all the matter that is. He knows it inside and out, He speaks and it is so. All things are possible with God. All things. The question you need to ask is, "Will God heal me?"
This question is posed to understand God's will, and with that will often times there can be timing and purpose. When one asks this question its not to see if He can, but whether or not He will do so. Again, His capabilities are not in question, but His willingness to heal is.
Now, this point may bother some people. It may even ruffle some feathers, but read it carefully. This isn't blaming the sick person for a lack of faith. It is simply making a distinction between hope and faith. A person can pray for healing asking God to heal them, and in this prayer there is hope. Its a hope that is relying upon God's present mercy as opposed to His established will.
What I mean is that some Christians seek after healing (hoping for it), as opposed to commanding it (in the name of Jesus). They are asking and seeking for something that is already provided through Jesus Christ. The thing is, a person needs to stand up, step out in faith, and command healing. This isn't so much a critique of the sick, as it is the minister. A person must understand God's will in order to have faith for its attainment.
So, if a person doesn't believe it is God's will to heal them they then can only hope God heals them as opposed to having faith He will. Often times I read people's comments on the threads here (over time) and its like a double minded man. One moment they say they have faith, and in the next they say that God won't ever heal them in this life. So then, is their faith God will heal them at death? Considering Jesus words, about your faith doing it unto you, thats a little scary, haha.
What if instead of thinking God can heal you, you think it is God's will to heal you now? Then you can have faith, because you expect the healing. You do not only hope to be healed, and believe in His capabilities but you also expect God to heal you because He has revealed in His word that that is His very will (through Jesus Christ).
The point being, where people often claim to have faith it is not faith. It is hope. Hope and faith while often accompanying one another, are not synonymous. I can hope God heals me or I can have faith that He will. There is a difference. Hope, while it may get results, does not have the same results as faith. Faith, expectation, and belief these things lead to results. Even so, God's mercy if we lack faith.
All in all, God is faithful. Learn His will, then believe.