I suggest that you try to find out what is wrong with it. Check with the GP and ask for referral for neurologist to check it out. We cannot diagnose for you in this site, but we can support you if you have dementia or other conditions listed in this healthcentral site at the left side in home page. It may not be dementia and it may be something else. It sounds like it is not pain but tiring. Please see your doctors about that. It may take some patience. e.g., I took a long while to find out why I am dizzy and etc. Sometimes it takes a  few professionals to find out what is exactly the issue. Take care, NC