i started playing guitar about 3 weeks ago and although i practice every day about 3-6 times day about 10-30 minutes each time i feel like i don't make any progress regarding any chord changes that include a open G. it's really frustrating since it's the only thing that hinders me from playing a whole bunch of songs. knocking on heavens door for example is as far as i am concerned a really easy beginner song, but i can't play it at all since i always mess up the change form the g to the d and then back from the am or c to the g.
so does anyone have any tips how to master the change?
First of all, welcome!

you really just need to take it nice and slow, just practice the changes themselves, don't worry about the whole song until you can make those changes. also practice playing just a G chord and getting into position for it until you can do it quickly without thinking.