What did Kayin mean when he declared that his sin was too large to bear?
Rashi: This was not a statement, but a question - 'If You can bear the upper world and lower world, why can you not bear my sin?' 1
Targum Yonasan: 'My rebellion is too large to bear, yet You are able to bear it!'
Ramban: Kayin was confessing his sin 2 . "True, he was saying, my sin is beyond forgiveness, but", he continued, "You, in Your mercy, punished me severely, but not with the death-penalty. Yet the punishment 'Na va'Nad Tih'yeh ba'Aretz', which leaves me no possibility of building a home for myself, or a pen for my sheep, renders me at the mercy of all who find me, and will inevitably lead to my death!"
Mishnas R. Aharon 1 p.20 (Hagahah): Indeed, this is true! It is harder to bear sin than to bear all the worlds. Great Chesed is needed for a sinner to receive his needs. Therefore, he is far from reward in the world to come.
This also seems to be the explanation of Targum Onkelus.
Mishnas R. Aharon 1 p.173: After the sin he was very afraid, just at the time of sin he had a spirit of lunacy. This was the level of the early Resha'im!
רש"י: גדול עוני מנשוא- בתמיה: אולי אמר זאת בדרך וידוי?
גור אריה: אם הייתה כוונתו להודות על חטאו לא היה לו לקטרג "גדול עוני מנשוא" שאי אפשר למחול לו, [אלא דבריו נאמרו בתמיה, שבודאי אפשר לנשוא עוונו].