In your case, no it would not be a mortal sin for you to eat junk food and break your fast. Also, God knows your intent, and if it was not your intent to not eat junk food forever, God will not hold you to that.
For a sin to be mortal, it has to have three criteria: that it is grave in nature, that the offender has full knowledge of the sin, and that the offender deliberately consents to the sin. In your case, it sounds as if the first two criteria are not met. But at the very least, it’s definitely not a grave sin, so it cannot be mortal. Even if it’s a venial sin, it’s not going to cause spiritual death. If you are afraid that your relationship with God may become injured because of this, express your love to him in another way. Say a rosary, or read all the way through one of the smaller New Testament books, or a Gospel chapter to sort of trade back to God, if that will make you feel better about indulging.
Remember God’s message throughout the Sacred Scriptures: He desires a contrite heart over sacrifice any day.