One regret I have is that I don't have the finances to take lessons, due to circumstances and time and such. Maybe I can in the future, so really I have improved through pure determination to succeed. Arming myself with knowledge and sorting through things. I'm with you that this is taking longer than if I had someone to observe my swing. I really think I would be shooting lower if I was coached.
I'm still convinced I can shoot to a single digit handicap as a self taught golfer for me it's a few keys areas physical as in training the body, technical the mechanics of my swing, course strategy smart club choices, and mental approach to every shot.
I'm working through this all by myself but thankfully the internet and books, and this forum are very helpful. The key part of this is taking information and implementing it into the swing. That's the part that decides whether you improve or not. I know there are other factors involved but getting the mind and body together and a lot of golfers could be single digit handicaps.
In my experience you may only need one lesson. In my one lesson I was given enough stuff to work on to keep me occupied for a year. Until you master everything from that one lesson, then maybe go back for further guidance.