First of all, I give you my respect for putting your voice out there. Whenever I listen to a cover, I listen for the emotion behind the words. Are you really singing as if you meant it, or just trying to sound good? In your case, I felt that you certainly did bring out the 'feel' behind 21 Guns. And that's where I think a lot of covers go wrong. In my opinion, the best covers are those that have been adapted and delivered to communicate emotions. Keep up the good work in this aspect.
Now onto the slightly more technical side of things.
One of the things that let you down in this cover is that your mic is clipping like mad when you get to the the final chorus, simply because your singing too loudly. It happens on several other occasions as well. I would suggest using a better mic, doing some mixing, or adjusting your microphone recording technique to avoid this (keep the microphone further away from you, reduce gain on preamp etc). Its not great to have the song seem like its exploding in your ears at certain parts. You're a tiny bit flat on some higher notes (something inside... 4:10), and you should also work on your breath control to hold the higher notes for longer.
Phew, that took a while to type up. Keep at it bro!