Mind Games

I have a question. Why do we let our mind or the devil, to play mind games with us and work up our anxiety? For example. I know i am behind on a credit card payment and I know for a fact that my credit card is over it's limit (i cut up my credit card a year ago, but due to the fact that my payments are smaller than the total interest being accumulated each month, it doesn't seem like i'm getting anywhere with it). I knew this in my mind, and then i got a call from my credit card company giving me a big speech about how i need to pay it --- as if i didn't know that! So now, I have myself all worked up and very much in a state of anxiety even though i knew this was the case to begin with. Plus the fact that i am paying interest on my student loans as i go along and those payments are not small by any means either. Being a student with no income to come my way for another month to a month and a half sure is stressful at times! Please pray that i just keep on trusting God with all of this and that my stupid mind and anxieties won't keep me up at night either.