I think this is a normal occurrence in the creative process. More importantly, I think it is an essential aspect of the creative process. If a composer were to like everything s/he created, chances are that composer would be not a very discerning individual, and their music would likely reflect that.
One of the skills that I think composers need to develop is discernment; the ability to evaluate whether idea y is worth pursuing or not.
The difficulty for most student composers, as I have mentioned before, is that their level of musical sophistication exceeds their level of compositional technique.
Why? Because most students begin formal training in composition when they reach university, but, in order to get into a Bachelor of Music programme, they need to have spent years developing skills in one or more instruments, often coupled with some music theory and history training as well. This results in the phenomenon of knowing that a composition, or section, or musical idea, is less than it could be, but not knowing exactly how to go about improving it.
The solution I typically recommend is to just push forward with your musical ideas, even if you are not convinced of their quality, because it is often only by doing this that you discover the potential of that idea to grow into something bigger, or at least something to which you can feel more attached.
It doesn't mean you necessarily keep and develop every musical idea you ever come up with; it just means that you often need to work with an idea a fair bit until you come to a better understanding of what it can develop into.
Should you ever discard your musical ideas?
I don't think so. For two reasons:
- If you have worked very hard on a musical idea, there is a good chance that it has value.
- You don't have to use it right away. You may find a use for it later, possibly in a different section of the same piece, or possibly in a different composition. You also may never find a use for it, but since we don't know whether we will eventually find a place for it or not, it makes sense to keep the idea, but just set it aside for now if you don't feel it works in the particular section of your composition for which it was originally intended.