I threw my Monte into park while driving it and then it started to shift late and now it won't shift at all, the RPMs don't even go up! Can someone tell me what is wrong with it? I already considered the transmission but I'm really hoping it's something else that could be easier to fix.

 

Its hard to diagnose something like this with so little data.

To be clear - it worked fine for some period after you did it, but then it started getting worse?

What do you mean that the rpm won't go up? The throttle pedal is completely dead?

What happens when you put it in the different gears? Is park truly in park? Does reverse work at all? Any forward movement in any of the drive gears?

 

Like Bumpin96 indicated more info is needed.
Why did you slam it into park while it was in motion? or did you accidentally bump it while driving?
These cars are very sensitive and need to be handled carefully. You may have gotten lucky and just broke something in the shift mechanism.
But if not it will be expensive You should seek out a professional garage on transmissions AMACO or one like that.