Read the Diary of St. Faustina Kowalska - Divine Mercy in my Soul.
saintsworks.net/books/St.%20Faustina%20Kowalska%20-%20Diary.pdf
It is available online for free, then you will know and understand the mercy of God.
Do you read St. Alphonsus Liguori’s work? if you do you need to stop. Because that’s what happened to me, and I changed my reading to St. Faustina Kowalska Diary,
St. Alphonsus is too scrupulous. Maybe the translator who is wrong, I don’t know.
Jesus loves you. I was in the same situation as you, but I read St. Faustina’s Diary and it changed my life completely.
Pray also the divine mercy chaplet which is taught by Our Lord to St. Faustina
Do not despair, do not fear, God loves you. His mercy is infinite. Even if you have had the sins of the whole world and the damned combined, you cannot exhaust his mercy. That is what St.Faustina said.
Go to confession frequently (weekly, or every fortnight, or monthly) that will get rid of your despair.
Despair is a sin, it is a sin of distrust.
There is no sin more hurtful to God than the sin of distrust, that is in the diary. You have to trust in God’s mercy. Say, Jesus, have mercy on me a sinner. Go like that until your despair dissipate. And say, Jesus grant me boundless trust in your Mercy, Goodness and Love, and forgive me of my sins of distrust, and protect me from sin of distrust.
Be careful of sin of distrust, because that sin can cause you to go to hell. And that’s what the devil wants. Trust in God’s mercy.
I was in the same situation, really. (because of reading St. Alphonsus’ work, and my many many horrible sins), and I said the above prayer and go to confession frequently. And the result has been miraculous. I am changed upside down.
Read the Diary, it will change your life. It changed mine, and gives me trust in God’s mercy, to know the mercy God has for me, and for you. He is but mercy and love. He will not do justice unless you push him too. People who go to hell go there by their own choosing by rejecting God’s mercy. But if we repent, no matter how big our sins are, God forgives and forgets. Especially when he forgives you in person, through the priest, in confession. Ask Jesus to give you the grace to go to confession frequently.
Jesus is present in each and every sacrament. But especially Eucharist and Confession. The ones we should receive frequently. Pray without ceasing.
May the merciful Jesus bless you and grant you boundless trust in his mercy, goodness, and love.
I pray for you
Rosaryguy, you are right, add the chaplet of divine mercy, and recite it three times a day, morning, noon and night, on a regular rosary
I say my rosary three times a day too, morning, noon, night. In addition to divine office and little office of the blessed virgin mary.
yourdivineoffice.blogspot.com/
fons_vitae, why don’t you recite the divine office also?
It is prayed 7 times a day. Every 3 hours. Excellent prayer
You can add occasional personal devotions, such as Chaplet of St. Joseph, Chaplet of st. Anthony
prayerguidance.blogspot.com/
The saints will help you keep away from despair
I have bouts of scrupulosity from time to time, and because of my OCD, I can get caught up in such things, so it really helps to have someone set me straight. If there’s any heresy I fell into when I was younger, it was definitely Pelagianism. 
if I had to choose a heresy I fell into, (besides being Protestant for a time 
) it would be Jansenism