Frustration and Depression

last year i got much tensed because of some weird thinking that went coming in my mind UN-intentionally regarding to deen and other religious matters. i had never thought of such painful condition because i really had a fear that not knowing, somewhere i may not get out of my deen. i fought and fought with such pain thousand times a day and made taubah each time bcecause i could never think of such bad things. it became difficult for me to combat this condition. i could not make i'tkaaf because of this frustration. now a year has passed, and i feel much better. but still i remain scary each time that again that bad thinkina will hit my mind and i will get disappointed. please guide ma in the light of shari'ah that what should i do to cancal out this fight permanantly so that i may get easy in making prayer. because i am too much scary that whether ALLAH will forgive or not.