Lately, I have had truly vile and false thoughts every time I pray or think of prayers. What should I do?

Respected Anonymous;

Based on what understand of the message of the BANI, you need to look at the subject of your prayers. Are you thanking the Creator for providing you all that you have, or, are your prayers asking for some other worldly needs?
Gurbani message is that if one’s prayers for anything else other than blessings of NAAMU, it is asking for troubles.
Please refer to the Gurbani verse
ਵਿਣੁ ਤੁਧੁ ਹੋਰੁ ਜਿ ਮੰਗਣਾ ਸਿਰਿ ਦੁਖਾ ਕੈ ਦੁਖ ॥
vin tudh hōr j manganā sir dukhā kai dukh .

So look carefully at the subject of your prayers and let us hear back from you.