It feels like I am at square 1 again. I struggle with strumming and fretting down barre chords when I stand up. I also saw that you can use your thumb instead but then again how do you mute the strings in way you can when you just use your first finger?
Yes it is, which is why Justin says you should practice the same way you intend to play, whether that be sitting or standing.
That said, it’s just down to practice. I regularly play both sitting and standing. It doesn’t take long to get used to the new position. You can also aid the process by using a strap when sitting, making sure the guitar is positioned in roughly the same position when sitting as when standing. (Though I don’t use this method and I do fine.)
I had too many thoughts so let me rephrase: what I was trying to say is that I mostly struggle barre chords when trying to fret it in the traditional way with the first finger fretting all strings. I saw that some people play the barre chords with their thumb being on the 6th string. I practiced this song naive from the Kooks where the song has this catchy chord progression whe you also have the scratchy sound with the first finger muting the whole fret
Thumb on the sixth string is a great technique that you would want to explore down the line but professionals who do that usually do not do it because they can’t play bar chords. It just fits certain play styles better. You would still want to work on your bar chords.
(There are always exceptions. KT Tunstall, as she says, created her entire play style based on avoiding bar chords as she found them difficult when she started)