Personally, if I did not confess some sins that were “iffy,” I would not feel properly disposed to receive the Blessed Sacrament and it would become a matter of great interior doubt and suffering for me. I do understand, however, the need for conciseness in a crowded situation like the one you described. My own method is this – do a very careful examination of conscience beforehand and try to summarize your sins, including the iffy ones, in as few words as possible. You do not need to tell the priest the whole situation unless it was a strange circumstance that affects whether you are guilty or not… truthfully, I do not see the harm in taking fifteen more seconds to quickly confess these iffy sins and then be able to go to mass with a free conscience rather than leave wondering if you have omitted something serious. If it helps you be faster about it and you have a lot to confess, make a list and bring it into the confessional with you. (Don’t forget to destroy it afterwards!!

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If you later remember a mortal sin that you forgot to confess, yes, you are responsible for confessing it, but if you have since received Holy Communion you are not guilty of sinning against the Blessed Sacrament. As for forgetting that a sin is grave matter at the time you commit it, then (as others have said here) it is not a mortal sin, because mortal sin requires a) that the sin is grave matter, b) you know the sin is grave matter, and c) you give full and willing consent to committing it.
Also I wish parishes would have more and longer times for confession (although it is hard to regulate because some weeks it seems like nobody is there). Saturday afternoon looks like the norm, at least for every parish near my house, which makes it extremely difficult for me to get there because I do not own a car and both my parents are using the cars on Saturday afternoons (I’m a convert, they are Protestant and don’t see the need for confession anyway and so it’s not a big deal to them). Does anyone else have trouble getting there at this time? So when I can go to confession it is a great joy for me and I see to it that I confess everything I am struggling with, unless there is an obvious long line and then I try to omit the venial sins.
It is my opinion that hearing confessions is a priest’s job and in no way should the confessions of the people be limited, however there are some things we just can’t regulate like how many people come. One more thing – some of us do struggle with sins that aren’t mortal and for these types of people it can be a great torment not to be able to confess those sins.
St. Faustina writes in her
Diary, “God demands great purity of certain souls, and so He gives them a deeper knowledge of their own misery…Sin depends on the degree of knowledge and light that exists within the soul” (112).
For those who are called to a higher holiness, it may be necessary that they either find a spiritual director to counsel them or set up a private confession outside of normal times.