Q: MBP thinks it's connected to an external display/Choppy graphics

so i bought my MBP 17" online from a random guy. Everything was working fine, till one day the vertical lines started to appear, and the MBP stopped booting. In short after a lot of sleepless nights searching for a solution (other than the Apple program, as my MBP doesn't cover that) i finally got one. which is to move some extensions (from system-library-ectentions). and finally my MBP booted. Which brings me to my next problem. Now the graphics is really choppy. specially in launchpad, downloads in dock, browsers and yes when i play videos. plus i can not control my brightness. i have tried using GFX card status to switch to the integrated display, which was a bummer as well. it says can't switch if there is an external display plugged. i have never plugged in an external display. So I'm guess my changing the extensions had something to do with it. I need a fix to this choppy graphics-not working brightness controls and stop my MBP from thinking there is an external display plugged in.

Im on El Capitan, the discrete graphics card is AMD Radeon HD 6750M 1024 MB. Please ask for any more information that might help to fix my issue!

Thank you.