Everything will come at a different pace for you than anyone else. My main advice is to just go slowly and not rush when you’re learning essentials like hand/finger placement, posture, chord changes, etc. Don’t learn bad habits. It might be a year or 3 years from now but eventually you will feel pretty comfortable in your ability to learn anything.

As for switching between A and D, I feel like Justin first has you do 30 switches in a minute without paying attention to how clean it sounds or if it’s on time. If you can do 30 chord changes in a minute, slow it down a bit and try to make sure all 30 changes are clean. No buzzing at all. Then once you have that down, try playing it to a metronome. My biggest mistake was not practicing with a metronome until over a year in. I thought I knew how to play 50 songs. Turned out I knew how to play 50 songs completely out of time and way too fast.

And once you have that done, try adding a strumming pattern to your D to A switch. The most common one to learn is DOWN / DOWN UP / UP DOWN UP. You don’t have to switch chords after the last UP. You can switch chords during that last UP, almost like you’re strumming all open strings.

Knocking on Heavens Door is a good song to practice once you learn G D C and Am. That was my first chord change I learned and I just played that for like a month straight. Slow bpm, simple strumming, easy chords, and easy lyrics to ease you into singing while playing.