To what degree would you say you believe what God says? 100%? Maybe 99%? Or perhaps you might say 80%.... You might even say lower than that.
What if - for one day - just for today - you acted like you believed everything God has promised you? Lets take Romans 8:28 for instance: "God causes all things to work together for good to those who love God, to those who are called according to His purposes."
Could you go through your day responding to every hurt, every disappointment, every dead end, every changed plan, every emotionally draining thing that happens as if you believed what God promises? Would even your thoughts agree with what God says? Would your heart sing His praises?
Would you be able to rest in the knowledge that God is causing your Parkinson's Disease (or any other malady) to work together for your good - indeed for the good of everyone who loves Him and who is called according to His purposes? If you REALLY believe that He means what He says and that you can count on it, then you COULD rest in belief.
What if you know that you don't have that kind of belief in your heart, but you want to trust Him - you want to believe Him? Then confess your unbelief to Him and ask Him to help you to believe Him.
Do you believe with Paul when he says "I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us." (Romans 8:18)?
What about Romans 8:35? "Who shall separate us from the love of Christ? Shall tribulation or distress or persecution or famine or nakedness or peril or sword?" Would you add Parkinson's Disease to that list and do you believe that's true?
What if you went through the whole day believing Romans 8:36 and acting like it? " For Thy sake we are being put to death all day long; we are considered as sheep to be slaughtered. But in all these things we overwhelmingly conquer through Him who loved us. For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing shall be able to separate us from the love of God which is in Christ Jesus our Lord."
What if - today - you acted like you believe God?