Ps. 73:26 My health may fail, and my spirit may grow weak, but God remains the strength of my heart; he is mine forever.
Our health gives out on us. It is a part of being human. As we age different parts of our bodies begin to fail us and we suffer with aches and pains. As well, sometimes our spirit may fail us. We encounter situations in which our resolve to overcome grows weak. Sometimes our minds fail us and we find it harder and harder to remember things or figure them out. It's just the way life is; we are all subject to it.
But, the good news is that God does not change. He is the same as He always has been. The same God who created all things by the power of His word, is the God whom we serve today. He can be depended on to be able to help us no matter how weak we may become or feel. The Lord will never grow weak or weary.
The Lord strengthens the heart of those who trust in Him. He forever will be ours when we trust in Him. We cannot trust our bodies, minds or our willpower to always be there for us, but we can always count on the Lord if we trust in Him. And the wonderful truth is, even if we utterly are unable to think or know anymore, if He is our God now He will be our God then. He never forsakes those who love and serve Him, even when they lose their ability to even comprehend Him anymore.
The Lord is our strength, both now and forever.