I don't know if it is possible for any creative person to properly evaluate his or her own work. I certainly am not able to do it. I can be proud of having written something that has all the pitches and rhythms in the "right" places, and if somebody likes what I have written, I can be pleased that it gave (or gives) him or her some pleasure. Beyond that it is really not my place to say whether something I write is "good" or not. Anyway I believe that the real value comes when somebody plays what I have written, and when somebody plays a piece I have written really well, I tend to give the credit to the interpreter. The music itself can only be what it is, and it is (thankfully, when I'm not playing) not my job to determine what that might be.
Every piece of music is incomplete until it is interpreted. Every piece of music can be interpreted in many different ways. Perhaps the best measurement of the quality of a piece of music is how personally it can be interpreted and still make linear sense.
Perhaps music making and playing is a bit like sewing. It takes a lot of know-how, patience, time, material, equipment, vision, experience, and talent to sew an article of clothing. That article of clothing is meaningless if it sits on a hanger. It can only be useful when someone wears it, and because people come in all shapes, sizes, and colors, that piece of clothing will look very different on different people. People also wear different kinds of clothing for different kinds of weather and different circumstances. It is my job as a composer to make sure that the pieces I write fit the "contours" of the people who might want to play them, otherwise nobody will.
I have clothes that I'm happy to wear around the house. I wouldn't wear them when I go out to dinner or out to teach. I also have pieces of music that I play every day and would never want to perform in public. Just because I'm not practicing something for a performance doesn't mean that it isn't a fully-functioning piece of music.
Composers today seem to put such a great emphasis on "getting" performances. I like hearing people play my pieces as much as any other composer, but I would be just as happy if the music I have written ends up mostly as something that people play at home with their friends, and without an audience. Is a piece that people play for fun and personal expression less of a piece if it's not being performed for others?