One more thing - my last teacher told me that much of my tendinitis and pain problems stem from playing too rigidly and holding much too much tension in my hands when I play. He showed me how that when you play anything on the piano, whether it's phrases or chords, your wrists should always be moving. Like when you come down on a chord, you shouldn't just sit there but you should move your wrist upwards. He explained to me that if you are constantly moving, then you are not as tense and therefore you are not overworking the tendons. I hope I explained that okay. I get what I'm talking about but maybe nobody else does. Oh well....my thing is to just remember all the 'good' advice when I'm actually sitting at my piano.
Your teacher is totally right. I'm so glad he is giving you good advice. It is amazing how many well-respected teachers out there are still of the old school of thought -- insisting on zero or minimal wrist and arm movement and certainly never any body movement. A very successful colleague of mine once told me that I should endeavor to move far less and sit straight up with my head centred above the back of my bum -- to imagine myself as if in a hospital bodycast that does not allow any back movement!!! Well, nothing is further from the truth when it comes to playing most effectively and with least tension.
I'll embarass you a bit, but hopefully for good reasons, by telling you that I followed the link to your Youtube videos so I could actually see you playing, Monica. I cannot tell just how padded/soft your bench is, but my suggestion would be, if you have a rock-hard bench with no padding, to switch from the padded bench to the rockhard one just for awhile. It would ensure that you are not sinking into the padding of your bench.
Take this as a nice compliment about your femininity, but to me, your position is too elegant. You seem poised very ladylike on the bench, which looks esthetically nice, but is not good if getting tendonitis due to the seating position putting strain on the wrists. You aren't a pound overweight, so your bum should not be taking up more than half of the bench's depth, unless you have a fairly small bench. I think you are sitting back too far on the bench. Instead, pull the bench out even further from the piano than it currently is, and then sit further forward on the bench. This will leave the back half of the bench entirely unoccopied if it is a large bench. It will encourage you to lean forward a bit. If you switch temporarily to a rock-hard wooden bench and play while wearing some shorts, the indents on the back of your legs from the front edge of the bench digging into them should be at least halfway up your thighs -- if anything, closer to where your thighs meet your bum than to where your thighs meet your knees. So try to have a bit less thigh on the bench than you currently do.
Hmmm..........can't think of anything else right now, but I hope that these things might help if you try them. I guess I tend to blame our childhood piano teachers for any current problems we have as pianists. I know they had the best intentions with the knowledge they had at that time though.