No one “always” has to commit a mortal sin. Ordinarily there are ways of avoiding the temptations that pull us toward sinful behavior. But first, we need to be motivated. Sin, after all is an action or actions that betray the love that Jesus has for us. I suggest that before all else, you spend some time each day looking at a crucifix and thanking Him for each of the sufferings He endured for us. Such consideration puts everything else in our lives into perspective. Then ask Him for the strength to be faithful to Him. Then avoid those thoughts and fantasies that tempt you. Make it a habit to just not go there in your mind.
If you should still fall into sin, go to Confession as soon as possible. Do not wait two weeks! I also suggest that you start spending some time before Jesus in the Blessed Sacrament. Also pray the rosary daily. Being a Christian means much more that just avoiding sin. It’s mainly about loving the One who love us the most.
You are in my prayers.
Fr. Vincent Serpa, O.P.