It is my understanding that everything including thoughts are essentially without substance, in that everything arises from emptiness and eventually dissolves back into emptiness. I have had many thoughts that I felt difficult, especially at 2 or 3 in the morning, and they and my ruminating over them are disturbing and keep me awake, and then they dissolve back into emptiness as I fall asleep once again. Actually that happened last night. For me, the ephemeral nature of thoughts is part of and the same as the ephemeral nature of everything -- nothing is always. My thoughts and my decisions come and go, like everything. For me that is not sad or negative, it is just how this human life is.