The difference is that in the rapid game you capitalise on your opponent's blunder. You do not play any better in rapid than in correspondence, your opponent plays worse in rapid.
Even at 30 minutes/move you should never ever blunder.
Besides there is no point in accepting correspondence at say 3 days / move and then take only 30 minutes / move.
If you have no fun in studying one day a move then correspondence is not for you.
"I do not menage in any case to see all possible variation"
How come, you have 3 days for your move, you are allowed to move pieces on the board. Of course you can see all possible variations to some depth.
"after a while I loose concentration and I take bad decision"
If in correspondence you lose concentration, then you can do something else and resume analysis later. That is the essence of correspondence. Correspondence requires no stamina, but it does require patience.
"I play classical because while waiting for my adversary I loose concentration and I'm no more in the game"
In playing classical you should not analyse while waiting for your opponent to move, you should only make general considerations. Only after your opponent moves you should revert to deep concentration and calculate all possibilities.
The difference is that in the rapid game you capitalise on your opponent's blunder. You do not play any better in rapid than in correspondence, your opponent plays worse in rapid.
Even at 30 minutes/move you should never ever blunder.
Besides there is no point in accepting correspondence at say 3 days / move and then take only 30 minutes / move.
If you have no fun in studying one day a move then correspondence is not for you.
"I do not menage in any case to see all possible variation"
How come, you have 3 days for your move, you are allowed to move pieces on the board. Of course you can see all possible variations to some depth.
"after a while I loose concentration and I take bad decision"
If in correspondence you lose concentration, then you can do something else and resume analysis later. That is the essence of correspondence. Correspondence requires no stamina, but it does require patience.
"I play classical because while waiting for my adversary I loose concentration and I'm no more in the game"
In playing classical you should not analyse while waiting for your opponent to move, you should only make general considerations. Only after your opponent moves you should revert to deep concentration and calculate all possibilities.