My Mac is exceptionally slow, what can I do?

Ok, heres the story. The day after christmas 2012, December 26 2012, I upgraded my Macbook Pro from Snow Leopard to Mountain Lion. I originally got the mac in 2010. The upgrade went by fine but after I noticed that the mac was slower than before, it would still function but there would be mild freezes every now and then. I was hoping that this was just the system possibly adjusting to the new software. The slowed performance never went away.

About a three weeks after I upgraded I got a new iphone 4 from att and it was mailed to me by att. When I recieved the iphone 4 I pluged it into my mac and transfered the info from my previous iphone 3gs to the iphone 4, then I put my music on it. I put all my songs in a playlist and then transfered them all onto the iphone at once. I left the mac and iphone sit since it was going to take a while. When I went back to look at the status of the music the mac was froozen and I was unable to get it out of it, I could move the mouse but it was the spinning wheel and I couldn't click on anything or quit or force quit anything. So I manually shut down my mac and started it back up.

From this point on the performance on my mac has been disasterous. With the Mountain Lion still on my mac it took over 20 minutes to start up from restart whether in safe mode, regular boot up, or resetting the NVRAM. Previously if I ever did have to restart my mac it would be able to start up within 5 minutes. Once the mac was running again, around every 5 minutes, almost routinely, the mac would freeze and come out of it after 2-3 minutes. Previously it would never freeze, unless I was running three movies at the same time or something like that. I could almost do nothing on my mac since it was always freezing. When I was finally able to start itunes thing would freeze almost immediately and freeze again almost every 3 minutes, even with everything else quit out of. I could not play any songs off of itunes and it seemed like every minimal task I tried to preform in itunes would severely freeze the mac, it would freeze for anything from opening the itunes store to pluging in the iphone to trying to play a single song. I would have to manually shut down the computer and go through the long reboot process again. After three days of researching what might be wrong (mostly through my iphone since the mac was froozen or taking two minutes to open a single page) I decided to erase everything on my mac and downgrade back to Snow Leopard.

After fully upgrading Snow Leopard, it ran better but it would still freeze often and be hard to operate. The freezes went from every 5 minutes to about every 30-45 minutes. Every now and then I would get lucky and it would take over an hour to freeze, but eventually it would still freeze. Things would get disasterous again when I ran itunes (again). Running itunes 11, the performance went back down to how it was with mountain lion without running itunes. Itunes would be better at running task but it would always freeze randomly every 1-3 minutes, again with everything else quit out of.

I downgraded itunes 11 to itunes 10.7 and things have gotten better, the routine freezings have stopped and now the mac only freezes when itunes is running, but it still severely freezes. I still can't put music on my iphone and the mac overall is still very slow, it took over 4 minutes to finally get to this page (exluding the signing in process).

Can someone please tell me what is wrong with my Macbook? I am currently running Snow Leopard again with itunes 10.7. I am about a week away from completely erasing my iphone and mac and trying to reboot everything again. If that does not work I am going to go get a cheap PC. I just want to be able to listen to music while I'm surfing online again and put my songs on my iphone!