If we attend a doctor s surgery and say we have symptoms when we actually have nothing wrong with us that is a false claim. If a man comes to your door claiming to be collecting for a charity, but uses the money for himself, then his claim is false. He has lied. So, if we claim to pray to God when He has not prompted us to do so, we are making a false claim. It is, then, false prayer.
Sincerity has nothing to do with it.
Our many false prayers are the result of a misunderstanding of what true prayer is. If we do not know what true prayer is, then we will continue making false ones. False prayers can be accompanied by feeling good, because we have been emotionally satisfied. But this feel good factor should not be accepted as being necessarily equal to the joy of offering true prayer back to God. All of these are ineffectual and false.