If i commit a mortal sin, but then go for so long without confession that i forget about it, and then i go to confession and confess all my other sins, but not that one because i forgot about it, am i absolved of that sin?
Yes, you are forgiven of all sins that you genuinely forget to confess - though not ones that you deliberately avoid confessing.
It’s a good idea to mention it next time you confess though.
You SHOULD confess it at your next confession should you remember it.
Are you required to confess venial sins at confession?
I think you are supposed to confess any sin you are able to recall.
We are not required to confess venial sins in Confession, but it is a good practice to go to Confession on a regular basis, say maybe once a month, and if we have no mortal sins to confess, our venial sins are also “valid matter for the Sacrament” and are also good to confess.
Keeping in the habit of regular Confession will help us avoid committing sins, as well as free us of any sins that we have actually committed.
If you have no mortal sins to confess you can confess venial sins, or even previously confessed sins if they trouble you even after absolution. I usually add to my confession the line for these sins and all the sins of my past life especially those involving…, I am truly sorry and ask forgiveness and absolution from God and the Church.
I go at least once a week and often more. Despite from what some on this forum will proclaim it is NOT AT ALL DIFFICULT to fall into a state of Mortal Sin.
Or maybe I’m worse than most people, because it sure is easy for me.
You are not alone…
I usually go once a week - regularly go more than that.