Hey OP -- good luck in your quest! Perhaps a little history will convince you to never, ever give up....
I have been a mid to high 3.5 player for over 30 years. But, I never quit trying to improve -- although sometimes my "improvement efforts" have had 2-year haituses due to other priorities (or rehab from the 5 knee operations I have earned along the way!) Needless to say, at this point in my life I play 90% doubles (100% in league play)
2 years ago I got "early start" rated to 4.0 after a very good 3.5 season (senior and adult) and boy was I excited -- after all that time! But, poor play at senior sectionals must have convinced the computer to knock me down, and so it goes.
It is common knowledge that playing up does wonders for your level (not to mention your game) so I talked my way onto a couple 4.0 teams, playing 3.5 only at the senior level (one of my best friends is the captain and we thought we had a shot at Nationals). Long story short, last year was another very strong 3.5 season, and I did OK at Sectionals although our team fell a bit short. Plus, playing 4.0 (doubles only), while I had way more losses than wins, most of the losses were competitive (except against a couple court 1 teams) and I felt I belonged. So, the early start ratings came out -- and again, 4.0. At 60 years old!
Perhaps this time it will stick -- folks in the know tell me the sectional results and decent 4.0 matches make it very likely to.
If I can stress 2 things, it is smarts and fitness.
Smarts means playing true to your style and always maximizing your chance to win points AND induce your opponent to lose points. My style can be maddening -- it is basically at all times to hit the shot that I believe best gives me the chance to ULTIMATELY win the point. Given my skill set, that means always get the ball back -- depth and direction are far more useful than pace at our level. When finally presented with a weak shot, I rarely go for the winner -- I just try to hit a 90%-likely-to-go-in shot that my opponent will have no good answer for -- either they make the error or they hit one back to me or my partner for an easy putaway. If I am in trouble, that is when the lob or blast works well -- I can no longer set up the point, so the shot I can make maybe 40%-50% of the time, which I would never use if the point is neutral or I am in control of it, becomes the only logical choice -- otherwise my opponent wins the point 80% of the time!
Smarts also means developing shots that 4.0's cannot pick on. One of the best is a dependable deep second serve with some spin -- whether a medium slice or something with more topspin. I found playing against 4.0's, where I felt no pressure to win, let me really go for a more solid second serve -- which I then translated to using against the toughest 3.5's. (I still wimped in the second serve against weaker 3.5's -- why take any chance on a double fault when I knew there was a better than 50% chance I'd win the point regardless of my serve's pace?) Similar logic with my service returns. I developed a pacier return to break out when playing against 4.0's who could comfortably poach and destroy my usual "just get it in" returns. While I made more errors, and did not need this return against weaker 3.5's, it increased my likelihood of winning points at 4.0 -- and against the top 3.5 players who were between me and a 4.0 rating.
Fitness is even more important. I started a regular workout regimen, and the benefit is less on the stamina to play longer points, but more on the strength to maintain proper technique under pressure. This probably helped my game MUCH more than any lessons or clinics. After playing this many years -- I KNOW what I should be doing, and can usually self-diagnose (not needing the precision which 5.0 and above players require). Unfortunately, I have let myself go a bit since the outdoor season ended, and so I am avoiding clinics until I get back on the fitness train -- you simply cannot sustain good technique if you are flabby, so why bother?
So, I hope you do not have to wait until your 7th decade to become a 4.0! But, if it takes you a bit longer than you would like, do not give up -- eventually your efforts will be rewarded.