I've noticed that when I'm spending a long time learning something like for higher level exams or performance I lose confidence in my ability to learn new music. Maybe it would make some sense to always have one piece on the go which is not going to take so long to learn but if I'm working on two or three bigger things I would feel overloaded if I had to do that. It also tends to make me feel as though really I'm not very good at all as I also find it difficult to keep any sort of repertoire 'live' while I'm learning pieces that need a lot of time and effort.
Has anybody else noticed this?
It's worse on piano than clarinet but I'm less advanced on clarinet.

I have a grade 3-4 adult pupil who, after a year off, wanted to start learning a whole heap of grade 4 pieces (not necessarily for the exam, just as a 'milestone').

After a month or so, she said she was feeling swamped (we'd started learning 3 pieces). I realised that it was because all the pieces were new, all were pushing the boundaries of her level and, because she'd taken a gap, there was no repertoire stuff already on the go, nothing to be polished up.

So, a couple of weeks later, now we're still working on the same stuff but each week I give her a new grade 1 piece and she has to prepare it as study/sight-reading practice the first week, then get it to performance standard for the next week and then it's dropped. So each week she's got one little new piece and one little piece that she's polishing up.

And she's happier because she's not just battling away the same music and feels that she can 'see' some progress, even if just on easy pieces.

Yes. I've found my adult pupils have specifically asked for some easier music.

So we have one piece that will take several weeks to learn and is pushing the boundaries of their ability at the moment. Then an easier for fun piece. And an exercise or two (we use dozen a day and laugh at the drawings!). Sometimes we might begin a new "difficult" piece just as the old one is nearly polished.

I also use this for my pupils who are maybe grade 2 or 3 level, although I don't often find them an easier for fun piece - maybe it will be a shorter piece. I try to ring the changes in the kind of material we use. I think that pupils (children) are easier to keep going, and are often satisfied with moving on with dozen a day regularly.

I haven't found it a problem since my teacher spelt out to me that 1) he wasn't expecting me to come back the following week with the piece almost perfect and 2) that he sees learning a piece as a long-term project. (I started learning the piece I am going to play in his end of year concert in June during the Christmas holidays, for example).

He claims to be able to see progress from one week to the next even when I cannot. I rarely have more than one piece on the go at a time and will quite often spend a whole lesson just working on a few bars of a piece.

I'm fine with that but I do find that it makes me feel useless sometimes that I only have whatever I'm working on. I don't often need to be doing anything more on piano but I'm trying to get together accomps for a clari student's exam and it's so long since I started anything new that it's hard to believe I can do so within the time frame I have. And the only way really is to pay little attention to my lesson stuff.
On clarinet I always have orchestra parts which are including some difficult ones now. As I'm having lessons I already have sufficient to work on so orchestra parts don't get a lot of attention.This is at least in part a concious decision as I see lesson stuff as having the purpose of helping me to improve in a sensible way. If orchestra parts are difficult I do my best. Conductor tells us to practice which I do a bit but some of the parts are not going to be up to speed for a long time even if I practised them as regularly as my lesson stuff.

Pieces have to be frequently refreshed to keep them in the repertoire. No matter what new material I am learning I spend a half hour each day playing through pieces that are at a standard fit to be performed, and if I find that mistakes have crept in I schedule some "repair time" for them.

The more thoroughly a piece was learned in the first place the less often it neds to be re-visited to keep it up to scratch.

I also find that technically difficult pieces (and technically difficult sections of pieces) stick in the memory better than "easy" material, as to master them takes closer analysis, fuller concentration, more systematic practice, and more repetition.

Yes i find it hard too. I find that especially after an exam, when there has been such an intense focus on polishing, that I almost have to relearn how to learn new pieces. Outside exam periods it is not so bad as my teacher keeps a fairly brisk turn over of studies so I usually have something new to learn every other week or so.

Keeping repertoire live is also a difficult thing. Having 30 minutes to review would be lovely but I really don't have the time (or the stamina on oboe) to add that to end of my practice. I usually try to give 5-10 minutes at the end of my practice session to looking at tricky orchestral bits. I'm planning to use this time over the summer to play through some of my "old" pieces to try and bring them back to life a bit so maybe that will help.

Above is all for oboe. Piano is a bit different for me (pending exam preparation aside) as I don't have lessons and just play when and if I want and either dabble in old stuff or have a go at random sight reading. I don't worry too much about playing for anyone other than myself generally.