I'd say a few things.
First, if you are Catholic, examine yourself and repent of your sins, regularly making a good sacramental Confession of them, including insufficient trust in God. The more you repent of your sins, the less you'll be subject to accusations and doubts of your eternal destiny.
- Surrender your life to Christ and entrust it to Him.
- Make following Christ and the Gospel the first priority in your life.
- Let the Holy Spirit transform your life, and pray for God's grace (and cooperate with the grace he sends you).
- Pray to trust more, pray to love God more, and keep your eyes on Jesus.
- Pray, "Jesus, I trust in you." frequently throughout the day, especially during your trials.
"Perfect love casts out fear" (1 John 4:18).
Say the Lord's Prayer at least daily if not more often. Pray,
"Lord, I believe; help my unbelief!" (Mark 9:24).
Establish a discipline of daily prayer, preferably in the morning.
"Cast all your cares on [Jesus], for he cares for you." (1 Peter 5:7)
In other words, when you face these trials, be open and honest and discuss them in prayer with God, asking for his help and grace and entrusting the problem to Him.
Thus strengthened, try some spiritual warfare when you get these thoughts. Just say,
"Spirits of anxiety and doubt, I bind you in the name of Jesus Christ and by the power of his blood, and command you to go to the foot of the Cross to receive your sentence."
Remember that having repented of your sins, you are no longer under condemnation (Romans 8:1). You are a son of God! Remind yourself of this. I also recommend getting a St. Benedict's medal to wear or keep on your person and getting it blessed.
Remember that any negative, accusatory thoughts you have, do not come from God, but from the Evil One, who is trying to discourage you and get you down.
Don't let him!