I hope others who struggle with this can give examples of things they have done to help them overcome this problem.
A very simple example I must admit, but it may indicate a way forward.
In general, my childhood repertoire is well-retained if it was something I listened to frequently -- Suzuki repertoire, major repertoire works, but not the violinist-composer stuff (Viotti, deBeriot etc.), and only some etudes. Things I've learned in adulthood, less so -- learned less thoroughly, though. Listening makes a huge difference; things that I know "by heart" in terms of aural retention generally stay in my fingers much more readily.
Stuff I used to know comes back easily with practice, though -- as long as I'm in sufficiently good shape technically.
So my advice would be: Listen to repertoire, a lot.
I think some of it has to do not with memory per se, but with confidence. I believe the works you think you're "forgetting" are actually in your memory. But you have a phobia that you can't remember them, and when you're put on the spot, nerves interfere with was was memorized.
I know a graduate pianist who said she didn't feel she really know a piece unless she had played it 1000 times from memory. Maybe better memory and confidence in that memory will simply require many, many more repetitions.
Ive been thinking about this a lot as my young girl has great problems in keeping old Suzuki repertoire memorized, in essense she sannot remember the old repertoire, it just does not stick. And I had problems in memorizing (Im not a pro, just an amateur pianist, but I could have become a pro had I wanted to).
Some people say that it is just a matter of repetition and in a way it of course is. But we require so much repetition to retain the past pieces that it is not possible to do. Both of us have a very good memory otherwise, certainly above average. Music is the only thing that I have ever had problems in memorizing.
One thing may be that my brains have all through my life produced their own tunes meaning that I can ” hear” my own partial compositions and the pieces I have learned mix with these tunes and loose their own integrity. And my daughter also seems to prefer playing her own tunes instead of just repeting the piece practiced. So this may be a part of the problem. I believe it is structural and not lazyness.
If you are not planning to be a soloist, what does it really matter if you need the music in front of you? Suzuki fanatics and those that memorize easily wont understand and if one is brought up with Suzuki then one may value memorizing skills very highly as some Suzuki teachers continually tell how important it is to play all the songs by memory (at this point Im quite sick of this actually).
I recommend following the advice of other posters and see how it works out, but don't make too big a thing of it, which could be counterproductive.
Again, not an exact quote, because I can't be bothered to find the video (pretty sure it's a newer one about the Real Book. Won't be hard to find if you're interested). That's essentially the jist of what she said, though.
https://www.youtube.com/watch?v=CJXnYMl_SuA
However when I have gone to try new instruments or bows I have assembled a brief agenda of musical fragments that contain the notes and strokes that I want to use in testing the items I will be trying. And that is the repertoire I have used in testing unless the items lead me to some additional improvisation.
The most reliable way to memorize music is photographic memory.
When learning a new piece, don't even pick up the instrument. Look at a small segment of the music, try to memorize the IMAGE of the segment and imaging playing it, with the sound of violin. After you have a good feel of it, put the score away, and imaging the image of the segment, and practice from reading the mental image. Work in small segments.
Seeing the image in your head is DIFFERENT from seeing the actual score. The image does not have to be razor sharp photos. Just vivid ENOUGH to recall the music.
Since you are memorizing an image, you can also include all the notes on the page.
Next thing you want to do is to LOGICALLY reduce the number of images you have to memorize. If the piece has ABA sections format, you need only to memorize only 2/3 of the music. Within a section there can be other LOGICAL arrangements that can help you to RELATE to photographic memory, such as memorizing the main theme, secondary theme, repeats, or chord progression. I call that theory memory.
Photographic memory may not be enough, you need to combine it with music memory, muscle memory, and theory memory. If you draw a blank image while playing, you can use music memory to "fill in" the blanks. When one type of memory fails at a given moment, you have another type to fall back on.
Because the brain will want to avoid hard work, and photographic memory the most difficult, you want it to be formed first. If you have memorized the piece with muscle memory first, it is more difficult to memorize it photographically.
Number each measure, and include the measure number in your photo memory. While you practice, you can randomly pick out measure numbers to play. That's a good practice to reinforce photo memory. If you want to go one step further, you can try to pick out random measure and play a few notes backward. You have to have photographic memory to play a few notes backwards. Practicing from random measures also help in recovering errors while performing, and recovering is a good skill to have.
Some other tips:
You may practice at a fast tempo to have more repetitions per unit time, but the very last run of the day should be a slow practice.
For large pieces/major work, break it up into smaller segments, avoid playing from the beginning to end too many time. I'm not saying don't practice from beginning to end. You need to make a judgement call on how you want to practice.
DON'T need photographic memory if reading the score is acceptable for performance, or pieces that can be easily memorized with music or muscle memory.
While many instrument playing abilities have to be formed when a person is young, and almost impossible to acquire after sixteen or so, but memory is not one of them, and requires no special talent. With some practice, almost any age can do it well, and it doesn't take it long either. Six months to two years should be enough.
Final note: there are international memory competitions, and the top winners can be a very forgetful person. They have to remember to use the tools to memorize the things they want to memorize. They admit they often forget what they want to get after opening the refrigerator door…
I am not a professional, and I do not have any real insight into how serious the problem would be for you. Certainly, if you are playing chamber music or piano-violin sonatas, you can always have sheet music present. Similarly, in an orchestra, you will always have the sheet music. So, it seems that the main issue would be either concerti or certain kinds of solo violin music such as the Bach Sonatas and Partitas. I do not know how you would be viewed if your career goal was to play that sort of music professionally. You will need to ask others who are more plugged into that world. Good luck!