I remember that a while ago I promised God to fast from junk food as penance for my sins. I don’t remember, but I might have promised him that I wouldn’t eat junk food ever in my life. I feel really bad about this now, because, while I promised to fast, I don’t know if I promised forever. I really want to be able to eat junk food on occasion, but I’m afraid of mortally sinning if I do.
Even had you promised as a penance for sin never to eat junk food ever again, it would not be a sin to set aside that chosen penance. Voluntary penances can be voluntarily set aside. Feel free to do so if you wish.
You might also consider contacting a confessor or spiritual director for help with discerning whether you have been experiencing scruples. Scrupulosity is an affliction in which a person unduly fears that he has committed sin. A confessor or spiritual director who has experience in helping those with scrupulosity can be very helpful in guiding those suffering from scruples. Your pastor may be able to make a recommendation.
Understanding Scrupulosity** by Fr. Thomas M. Santa, C.Ss.R.