I just joined the forum to make a post in this thread ... I feel like I have very recently gotten over the plateau after 20 years of skiing (wow!).
I live in Texas, so the opportunity to ski locally is pretty limited :-)
I started skiing about age 14 on some family trips. I was the only one of my siblings to really like skiing, so our trips were pretty limited. About 3 days of skiing per year for the first 5 years or so. I took lessons and progressed to green/easy blues in the first 2-3 seasons with trips being mostly half day morning lessons with afternoon free ski. To become a solid intermediate (i.e. any blue), it took a season of more days than usual to really get there. I found that you need a certain number of 'miles' to become confident in the equipment before you can become a true intermediate. I also needed more than 1 trip in the season to really become competent at skiing blues. I had a season of three trips, and by the end of that season, I felt very comfortable on any blue and could get down double blue/blue black terrain without too much trouble. I had to put it more time to progress.
Over the next 10 years up to my late 20s, I managed about 3 days a season on average. Sometimes I missed a season. Sometimes I made two trips. I was in the intermediate plateau for sure. I skied with other skiers in the same plateau. I could do bumps, but it was for the challenge of it. I could manage 2-3 black runs a day, but more than that and I wouldn't have the energy to ski the rest of the day on groomers. I rented all gear. It didn't seem worth it to own anything other than a helmet. I worked out in the gym to keep fitness up, but not with a trainer. Part of my exercise motivation was skiing and getting better at it. Running and weights, little stretching, little core work.
I took a smattering of lessons over the years as an intermediate. I did a bit of bump work, but progress was slow. I came to the conclusion that I wouldn't become good at bumps without spending a lot of time working on them. I practiced them whenever conditions were good, but felt stymied and accepted where I was. It was hard to make progress when the first day or two each season was spent getting back to where I was the last season.
Starting at age 29 (6 years ago), I started getting bored with my usual exercise and started doing group exercise classes. It was interval weight training format and it really pushed me in areas of core strength, flexibility, and balance. The next time I went skiing, it was incredible. I was in much better shape than I had ever been in before, and the exercise had paid off hugely in skiing. I could do more bump runs, but my technique wasn't really any better. Improved balance did help some. I was still only managing about 3 days per year, but the days were so much easier. I was making progress in bumps only in the sense that I was very familiar by this point with their shaping, layout, what a 'good' bump looks like vs. bad. I was developing terrain reading skills but not really getting down them any better. I was brute forcing it, and I could because my body could take the punishment for 3 days. I was not absorbing bumps well.
This past season I finally made a breakthrough. This is what I did-
-continued exercise classes emphasizing core strength, flexibility, balance, cardio
-3 day ski clinic in December 2015 focusing on edging/carving
-purchased my own boots (using some tips on this website)
-2 day ski trip free-skiing to really practice the 3 day lesson techniques (Feb 2016)
-6 day ski trip with a ski club with 1/2 day lesson (Feb 2016)
-reading more about ski technique for bumps
What it took to break out of the plateau was some instruction early in the season, good physical conditioning with balance skills, and a lot of ski time to play with earlier good instruction as well as things I have read. The boots have made a big difference in the amount of control I have now and it is very noticeable in the bumps. I couldn't tip my skis down over bumps before like I can now.
I had to develop trust that the skis will be there to progress to dynamic carved turns (required for advancing). This is really hard to do with limited ski days and I assume it depends on personality as well - some are more fearful. I think people get stuck in the plateau because they never develop trust in the ski to be there and/or cannot get over the fear factor. Working on balance outside the ski season has also greatly helped me feel solid on my skis. In addition, my own boots have definitely improved control and the sensation I feel skiing is familiar each trip instead of alien the first day.
If you already have your own boots, good balance, taken a bunch of lessons and are still stuck, I recommend picking a season and make it a priority to ski a lot more than normal that season. There is a lot nuance in advanced skiing that I don't think you can develop without really putting in a lot of time with multiple trips/days. Some quality instruction is good, but you need multiple days to practice it and reinforce it. You also need multiple days so you can relax a bit more skiing and not be as concerned about maximizing runs skied, and instead focus on improving.
I hope this helps others who are stuck...