Back up your phone using the phone's backup feature.
Reset your phone using the phone's factory reset feature. Make sure that the battery is fully charged before you do this. (Why do a backup first? A reset will erase all of your user data and downloaded apps. It will restore the phone to a factory default condition. It will be like when you first bought it).
Then see if your BT function is working properly.
If it is still not working properly most likely you have a hardware issue with the phone. If you determine that it is a hardware issue, check whether the phone is still within the manufacturer's warranty period. If it is make a claim for a warranty repair or replacement.
If it does work properly, then using the backup you made earlier, restore your phone using the phone's restore feature.
If the BT is still working properly when you have restored the phone to what it was before you started all this, then the reset has resolved the problem.
If the BT does not work properly after you have restored it from the backup then perhaps there is a downloaded app that is causing the problem. The hard part is to find out which one. You will most probably have to uninstall your downloaded apps one by one and try the BT each time until you find the one that's causing the problem. Start with the ones you think were installed around about the time you first noticed that you had a BT problem.