LG Optimus F3 MS659 secure booting error, cause: boot certification verify.

I keep getting a message on the LG startup screen, that says,
"secure booting error
cause: boot certification verify"
My phone was rooted using king root almost 6 months ago, and I had no problems with it. Until around a month ago, it started getting real slow and apps would randomly stop. So I decided to format the phone. There weren't many problems after that. but after a while it started getting slow again, but slower than the first time. One day almost every app closed and "LG Home" would close, it would give me some kind of error. I turned it off and tried turning it back on, but that's when I got that error. At first i thought my phone was bricked, i still sort of do, so I tried several methods. I can only turn the phone on in download mode, and my computer recognizes it as an android device
At first I had some problems because i couldn't find my right .KDZ file, I used KDZ Update and I tried using LGFlash tool, but i can't find any of the files (.bin . dll) I need. I've also tried using the 2014 version of the LGFlash Also used the LG Mobile Support tool, and nothing! They all take around 4 to 5 hours to finish!!! It never says there's an error or anything, none of them do! They always finish successfully, but the same thing keeps happening. Every time the phone reboots, it gets stuck on the LG logo and i get the "secure booting Error"!! What else can i try? Any suggestions? Am i missing something? All drivers are installed correctly and I've tried on windows 7, 8 and 10! Is there anybody who can help me? What do you think the problem could be? What causes the "secure booting Error!"??

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