To be honest...it sounds as if you'd be much better off buying Logic Pro. If you feel that Garageband is holding you back...then you'll never get that doubt out of your head. And frustration is a real choker when it comes to doing anything creative.
I think I'd definitely get my Mac looked at too...it should be able to do all of that in it's pyjamas, really.
I upgraded to Logic Pro a while back, but I reverted back to Garageband. I just couldn't be arsed with how complicated everything can become. I think my songs sound better now...despite being done on a very simplistic DAW with no third-party plugins...and I certainly have more time to play, write, record and go and do other stuff than I would have had if I was sitting looking at a icon-salad monitor screen, scratching my head trying to figure out all the extra features I'd dropped £200 on.
My tuppence worth...