iPhone 4 won't turn on or restore

I pulled my jailbroken iPhone 4 out of my pocket about a month ago and it wouldn't turn on, plugged it into the wall and the Apple logo came on and off, held the power and home buttons and a message came on telling me to connect to iTunes.
I connected to iTunes and it didn't connect or anything. Threw it into recovery mode, tried to restore, and got Error 21. Tried DFU mode and the same thing happened. I let my boyfriend mess with it and he got rid of Error 21 but replaced it with another error, got rid of that and got another, now I'm messing with it again and am stuck with Error 1602 (now when I plug it into the computer the Apple logo shows after a few seconds without having to press any buttons, then it goes away, then shows again, and repeats.)
I watched a video with a fix using DFU mode and Tinyumbrella to restore SHSHs, but TU won't recognize my device when I plug it in, put it into recovery, or DFU (I'm not sure how this all works so if I'm being stupid about stuff please don't be a jerk about it.)
I tried to manually input my ECID into Tinyumbrella but after I hit "add custom device" nothing happens.
I'm completely lost. No idea what to do. Please help if you can!
Thanks:)