Why is my iPhone keeps shutting down and won’t turn

So I currently bought a 2nd hand iphone. everything is pretty well working and the only problem was the touch id. the seller said that it was because of the update so i let it be. but as days goes by, my iphone screen starts flickering like on and off in a continuous manner and this will only stop if i press the back part of my iphone and it will only start again if i press the back part of my iphone. i thought that’s the only problem but when i was at school and preparing for our roleplay my iphone just went dead and no matter how hard i try to open it, it just won’t open. so i tried charging it for the whole day at our classroom but it still didn’t open so i went home and tried charging it again and then it opened with a 100% bar just like when it went dead. so i thought it’s normal so i used it again and then the next day it went dead again and i’m not even using it. the screen went too dark on the left side and too bright on the right sad and it won’t stop shutting down and i’ll leave it overnight for it to open again. i don’t know what to do. i already reset, hard reset and restored it but it didn’t fix the problem. i went to a technician and they tried replacing the battery and screen and it still won’t work they said that it is PROBABLY the u2 ic of my iphone but they were not sure they only said PROBABLY and i don’t want to spend so much money when they’re not even sure. what is wrong with my iphone. please help :(( i’m just a student and i don’t know what to do. please help me fix this. thank you so much!!