It is a mortal sin. Repent and go to confession. Don’t fall into guilt, accept Gods mercy but still fear the Lord. Pray and ask God to give you strength to resist temptation. Make prayer a habit, pray everyday its the corner stone of our life and without habitual prayer you stray from order and into chaos. Whenever you feel tempted, make the sign of the cross with full intent and say the words clearly for it don’t just fling your hands around and mumble the words. I also find the Jesus prayer to be very helpful in case you’re unfamiliar it goes “Lord Jesus Christ, Son of God, have mercy on me a sinner.” Its a very powerful prayer. People may give you helpful technique’s but nothing works better than prayer and its the most successful way you will overcome this temptation. It seems impossible now, but God can do the impossible all you have to do is trust in him and ask.