I have a 97 2.2 cavalier with about 145k miles on it and it started overheating a couple weeks ago. I took it to a shop and they said the water pump was out. I replaced it myself, along with the thermostat and the car ran fine for a couple days. Now the coolant light is back on, very low on coolant and its overheating again. I haven't seen it leaking anywhere so I don't know where to go from here. Any help is greatly appreciated.

Check you're cooling fan ,is it turning on ? And make sure you bleed the cooling system properly.
Test you're cap and last ,do a leak test on engine to make sure you're head gasket is in good shape

Fan works, bled the system and checked the cylinders. Coolant in them along with milky oil. The A/C and cruise don't work in the car and theres rust on both quarter panels and rockers. 145k miles and only gets about 18 mpg. Is it even worth fixing or should I just sell the car?

In my opinion...its a shame with that millage to see these engines fail like that.
If you go a do the job yourself its worth it... maybe 300 to 400 on parts,machine shop
head gasket,gaskets,water pump,timing belt etc...it depends of the extense of the damage and if the head is not cracked ,guides,burnt valves etc,etc
But if ya have to pay someone else to do it ...its go a be a $900 PLUS job ,
Is not worth it to dump that kind of money in these cars