car just started failing after about 120,000 miles on it. i got it when it had 99k

right now its at 150k and i had the whole engine rebuilt because it kept leaking water, overheating, and the reason i rebuilt the engine was because it had a very rough idle. thats why my mechanic put some sealant because he thought it was the head gaskets that needed to be sealed causing it to shake when in a stop, it all caused the cooling system to clog and i had the water pump, thermostat and radiator replaced 3 times!! so i finally did the whole engine so we could flush out the sealant. it hasn't overheated since, its been about 5k miles but i no longer feel confortable driving it to far distances, only use my car from home to work and thats it. every day i look at the dashboard hoping its not overheating. yet after all the money spent on it and rebuilding the whole engine which was about 2k (and thats because my brother is a mechanic so they got it done where he works so he got a good deal, otherwise it would've been about 1-2k more) so after rebuilt it still has very rough idle. when im in a light it shakes so much i feel like it will lose power. it sometimes give me the check engine and when i use the computer to read it it says cylinder 4 misfire. we have tried everything, different mechanics but can't get it fixed. so cylinder 4 misfire still there after about $4,000 in repairs. very frustrating. why do i keep investing it in it you may ask, because i like the car and every time i have hopes that it will work ok now after everything has been changed. hasn't broken down again leaving me in the middle of the street but the cylinder 4 misfire still there. if anyone else has a fix for this please help!