I have had this problem for a HUGE while! I've looked it up on google and came up with 4 good solutions, all from the buddies at "MacRumors" (the mac forum).
SOLUTION #1: Resetting your display.
1. With your computer on as normal, press COMMAND, SHIFT, ESC, EJECT (the key for taking out CD's).
2. Hold those keys until your screen goes black.
3. When it goes black, release your fingers from the keys.
4. When the screen goes on again, your mouse should go back to normal (it did for me).
*This solution resets your display. IDK why it works for making your mouse go back to normal, but it does.
SOLUTION #2: Resetting your computer (it doesn't change settings or anything like that).
1. Take out your battery (if you have an older macbook).
2. Take it out, set it aside, grab the power cord.
3. Take the computer (without the battery or the power cord plugged in) and press the power button for 10 seconds.
4. Then, after that, take the power cord, plug it in the computer (without the battery in), and turn on the computer.
5. As soon as the light goes on, press Command, P, R. (This resets the PRAM)
6. Hold those keys until you hear the start-up chime 3 times. Let go of the keys.
7. Turn the computer off again, stick the battery back in, and turn it on again normally.
*This solution resets something. I (and the people at MacRumors) don't know what it resets, but it does work for making your mouse go back to normal!
SOLUTION #3: Resetting your PRAM.
1. Shut down your computer like normal.
2. Restart it like normal, and as soon as the light comes on, press Command, P, R.
3. Hold these keys until you hear the start-up chime 3 times. Let go of the keys.
4. When the computer turns on, log in like normal. Wait for everything to load.
5. Once everything is loaded, shut it down and restart it again.
*This solution resets your PRAM. Don't know why it helps with your mouse behaviour, but it does!
SOLUTION #4: Booting it into Safe-Mode, then shutting down & restarting again.
1. Shut down your computer.
2. Press the power button.
3. When the grey screen appears, immediatly press SHIFT.
4. Hold it.
5. The grey screen should now have a grey bar at the bottom that is loading. Keep holding the shift key.
6. Once it's loaded, it should take you to the normal login screen, only this time it will say "safe mode". Log in like you usually do.
7. Once you're logged in and everything is loaded, shut down and restart your computer again like normal.
8. Log in again.
*This solution should help, it apparently resets something. I (and the people at MacRumors) don't know what it resets, but it does work for making your mouse go back to normal!
These solutions should help! Try every one of them (please do). If they work, or if they don't work, please reply back and tell me! Thanks!