Hello everyone!

I play guitar in the last two years, but I don't really closing to the level I want to achieve. For example I'm really struggling with palm muting, especially fast palm muting.
Sometimes I can play the songs as fast as I should play, rhytmically correct but sometimes I just can't do it. The latter is more common and I don't know what should I do to build up speed.

I try to play riffs from these songs:
(sometimes work as expected, but I can only play this always the same way if I do "pm . . pm . ." and not "pm . . . pm . . .")
(only rarely work as expected, but I can play this too if I miss one palm muted note. I think I should not miss any notes, my brother just miss it if he can't play it normally... btw I'm talking just about the riff from 0:55, I don't tried the solo or others)
and sometimes I mess up this riff too...

I don't know what is the cause, but sometimes I just can't keep the rythm, sometimes I can. I don't used metronomes in the past and I don't really know how to use it effectively... I don't know that I should only practice and I will be ok with this or I should change something in my technique...

I've found this:
I used to mute in the normal way not this one but sometimes I tried this myself in the past especially when I was tired. What do you think? Is it worhty to play those things like that?

Oh, btw if I don't have to change between muting and normal unmuted downstrokes, I can play better. I mean riffs with only palm muted parts and no changing between muting-unmuting is not as bad as these.

I want to know everything that comes in handy when doing mutes
-How to hold pick
-Where is my hand
-Should the guitar up towards my chest or down on my stomach is good enough
-Exercises before playing?
-Exercises to build speed?
-Should I play against the metronome?
-down-up strumming while palm muting? (I do this only for one string riffs not for chords or power chords - those I play with downstrokes only)

Tell me everything that can help
Basically, you want to build up speed with palm muting just like everything else. I sort of skimmed your thread because I'm lazy today, but I gather that you're trying to do them fast right out of the gate. Stop that now. You're only hurting yourself.

Now, as far as holding the guitar, pick, etc. It's all a matter of choice. Obviously, you don't want it hanging down by your ankles, but having it high on your chest isn't a bad idea, especially if you want to get into shred later. MOST of the best shred guitarists hold it pretty high. However, there are a few that make the lower area work Eg. Paul Gilbert. They hold it high up because of their fretting hand, mostly. The wide stretches are easier to make.

I hope this helps. I know I left some things out, but if you have any more questions, feel free to ask here or PM me.
If I understand your problem right, it could be that you naturally put your picking hand in a position that makes it hard to switch between muting and not muting, by making you move it a long way whenever you try to switch or in some other way.

Try and avoid anchoring your pick hand on the guitar when you play, keep it free to move.
Up The Boro!