2003 Nissan Maxima

Do you have an idea where the coolant went? Refilling every few days meant that you're loosing it somewhere. Things to check:

1. Is the engine bay full of white spots due to dried up coolant spraying engine bay? If so, you need to find that leak and fix it ($200-$500).
2. If the leak is not visible anywhere (radiator/hoses), you may have a bad head gasket, causing coolant to be ingested into the engine cylinders and burned off when the engine is running. You would then have plenty of white smoke from the exhaust pipe. This is a bad case scenario and will cost over a $1000-$3000.

The fact that you can't get above 30mph is probably because of the bad head gasket. You may soon will not be able to start the car because the cylinder will suck up too much coolant, a problem called hydro locked.

A small possibility is the the lost coolant causes overheating and thus the engine can't run at normal temperature, thus the low speed only capability.