May God bless you! I wasn’t in the same situation you were in but I used to withhold embarassing mortal sins not knowing that you needed to confess them for your confession to be valid. Eventually, I made a general confession (confessing all mortal sins in my life by kind and approximate number) and felt a
lot better since. My faith has grown abundantly since.
Some advice: You must confess all unconfessed mortal sins that you remember by kind and (approximate) number for you confession to be valid. Don’t think you need an exact number for your confession to be valid. Think of it more like going to the doctor to be healed. If you’ve had migraines for the past six months there’s a difference if you had them almost daily or almost weekley. If you confess to a priest that you’ve been viewing pornographic movies you don’t need to give him an exact combined running time of all the films you’ve viewed for your confession to be valid (unless for some bizarre reason you have such a number). Saying that you’ve viewed about two or three pornographic movies a week for about the past four years (or saying something like around 1500 pornographic films) should be sufficent. The same thing with pornographic images. Giving an approximate number of images (around 5,000) or viewing habit (for about five years spent between a half an hour to an hour daily viewing pornographic images) should be sufficent. Also, any mitigating circumstances need to be confessed. For example, stealing $100 from Bill Gates is different than stealing $100 from the Chruch. So if you stole from the Church, that would have to be confessed.
Now, God doesn’t expect you to be perfect and confess sins as if you’re a computer printing out numbers. Just do your best to confess all mortal sins you remember by kind and (approximate) number to receive valid absolution. After you receive absolution ALL YOUR SINS ARE FORGIVEN! If you get in your car and suddenly remember something that you forgot, it is forgiven! The only thing that can drive out sanctifying grace is another mortal sin. Remembering a mortal sin you forgot to confess will not put you back into a state of mortal sin. Just bring up any remembered mortal sins at your next confession. Receive Holy Communion as long as you are in a state of sanctifying grace. However,
do not start looking back to see if you confessed everything. Accept that God has forgiven you and if a past mortal sin pops into your mind just bring it up at your next confession.
Also, I highly recommend the Divine Mercy Devotion:
ewtn.com/Devotionals/mercy/
and praying three Hail Mary’s in the morning and evening to preserve you from mortal sin during the day:
catholic-church.org/grace/marian/3-hail-marys.htm
One last thing: you need a firm purpose of amendment for your confession to be valid. For example, if you’ve been having an affair for the past three months and go into confession thinking “I’ll confess it but I don’t want to stop it and I’m not going to try to stop it” then the confession would not be valid. If however you go into confession thinking “I don’t know if I’ll be able to stop having the affair but I’m sorry for having it and am going to try to stop having it” that would suffice.