Hey verwel,
I think I know what you mean. The piano itself starts to sound dull and "null", after hearing compositions for orchestras or just multiple instruments adding their own strength to the whole.
This can also happen if you have a routine for playing but forget why you do it. You're supposed to play for your own pleasure (I don't need any kind of commentary to this one) and if you cease to "understand" why music pleases you - not necessarily in general but forget the meaning of all the details here and there - the music seems to turn out to be empty.
For example, with me, take Händel. I'm sure he's a great composer and all but I can't stand his music with violin involved. I always feel the need to get moving during listening to it so I turn it off and still haven't been able to get familiar with it.
Once I stumbled upon Camille Saint-Saëns' Danse Macabre and was totally moved by the music. So later on I found the piano transcription of it and loved it equally or even more from certain perspectives. I had already got to know the piece in all of its instrumental dimensions so hearing it on piano just made things beautifully "simple".
Maybe you should drop listening to music for a little while, or at least drop the piano pieces. Then, after a while, go sit at the piano and get to know it - again. Like you were at it for the first time ever, press the keys carefully at first and listen to what it has to offer. Going slowly helps to build up a relationship of respect. If the piano doesn't seem to have anything to tell you, find new music, there are lots of less-known composers out there who have created some brilliant music.
Also, there are piano transcriptions. Take Wagner's Ride of the Valkyries for example. Maybe if you get familiar with the emotional side (Wagner's compositions are often romantic) of a piece, you will be able to express its colourfulness and emotions through a piano alone. Maybe try also Liszt's Totentanz (danse macabre), its a great composition and has been transcribed for 1 and 2 pianos!
I have no idea if there was anything valuable in this post for you (who has played piano for 25 years) but I am sure that you're not "doomed".

We just never get to know our friends perfectly, and this could be the case with an instrument. There's a lot more to it than just tapping the keys and listening. Go experiment and desecrate old believes/patterns on purpose.
I really hope you'll get back to it, that much of a wonderful instrument a piano is to interpret.

p.s. Sorry if my english is inflexible and annoying to read.
