Stray thoughts

Sometimes you have this feeling that all what you are doing in the name of Islam is just for show off (to people) and sometimes you are not too satisfied after doing a good deed like praying, charity or any ibadah for the fact that you think all what you are doing is just hypocritical. You never have this feeling of sincerity to please Allah anymore. You always look good on the outside (for e.g you pray 5 times daily very often in the Masjid, you fast optional fasting, etc), but deep inside, very deep inside you, you have this weakness and very dangerous thoughts about God and this brings unhappiness and guilt in you. Please how can I get rid of this and what is the root cause of this?