Temptations of the thoughts are often the hardest to fight, because they seem to come from within. But have hope, for Christ dwells in you, and "He who is in you is greater than he [the devil] who is in the world" (1 John 4:4) Here are a few suggestions:

1. Pray the Jesus Prayer: "Oh my Lord, Jesus Christ, Son of God, have mercy on me the sinner!" Keep repeating it until you are truly and sincerely meaning every word of it from the depths of your heart.
2. Memorise and pray a favourite Psalm, especially a psalm of humility such as Psalms 16, 51, 130 or 131.
3. Pray the simple prayer of the tax-collector: "God, be merciful to me, a sinner" (Luke 18:13)
4. Sing an inspiring hymn such as "O Seeker to meet Jesus", "The eve of the Last Supper" or any other hymn that moves your heart with love for God.
5. Learn and sing one of the beautiful Church hymns from the mass or the tasbeha. "We follow You with all our hearts (Ten-oweh ensok khen pen-heet tirf)" is a good example.
6. Memorise and repeat a powerful Bible verse, such as Philippians 4:13 "I can do all things through Christ who strengthens me", 2 Corinthians 12:9 "My grace is sufficient for you, for My strength is made perfect in weakness" or Micah 7:8 "Do not rejoice over me my enemy, for if I fall, I will surely rise".
7. If the bad pictures come back again after you have finished praying, just start praying again until they go away. Make the devil get sick of tempting you that way!

Find out which prayers work the best for you. You may even find that different prayers work better in different situations. Of course, you need to discuss with your confession father all the other strategies you can use such as avoiding tempting situations, fasting, metanias, good influences, keeping your mind occupied with good things, regular repentance and communion etc.

Never lose hope! Millions have successfully gotten through this before you!