I think you have a point, Penny, in that most of us reach a plateau at some point and find we don't easily make much more progress. To some extent it's just something we have to accept, although I daresay there are instances where lessons would help. The level you can reach depends on your abilities but also on the choice of instrument.

I am a weak guitarist, but my deficiencies are the other way round from yours. I can ususally work out what chord I want, but am a bit clueless as to what my right hand should do with it. Maybe we could work out some kind of double-act :-)

Seriously, if you can play in G, you don't need to learn much more to play in D. Learn the chords of A and Bm and you'll find that, along with the chords you already know, you've got the basics. With those two keys and a capo, you should be able to strum in any key you want. (Proper guitarists, shut up please. I know this is not what an expert would do, but this is from one non-expert to another, OK?)

But a word about keys (and I do know what I'm talking about here): no one key is right or wrong for your voice. It depends entirely on the range of the song, which - in G for example - might run, say, from low G to middle-B, or from middle-D to high D. You need to find the right key for your voice for each song, either by trial-and-error or by knowing your comfortable range and finding which key will place the song there for you.

Marje