You mentioned that the bombs knock you out. Do you know that you can shoot star bits at the bomb to blow it off before it hits you?
Like another mentioned get the Mario galaxy guide book or watch your levels on youtube from different players. I was in the same boat as you thinking I could never complete the game and was sad since i was having so much fun. But i stood in there and now i am playing as luigi with 64 stars.
All i am saying is this game is fun and you just need to know all the things that you can do in the game.