Q: overheating leaking don't know what the problem is

I have a 2002 Mitsubishi Montero and for a couple months have had a small coolant leak. a couple weeks ago it started to overheat and I noticed there was no coolant in the system. I filled it with water and it was fine. slowly it started leaking more and more and I would have to refill more often. when it was low and overheating the engine would cool down when driving faster. yesterday for the first time it started making a squeaking noise and within two miles of driving it overheated to the max. when I came to a stop and car was idling the engine slowly started to cool. when I started driving again faster it got hotter and was boiling. the leak is a little more major noe I will find huge puddles instead of small ones and more so when parked on a hill. today the car doesn't want to stay started unless I keep my foot on the gas. while driving the heater doesn't even cool it down. please please what could this be??

My car has 214 miles.
My car has an automatic transmission.