In my case the usual reason they're demotivating is that I usually know that they think I can do it; they're just spelling out their model of me. Usually the model of me is so bad that I'm led to further discount their opinion, but they're signaling that they care which makes them more likely to be painfully disappointed in me. Basically those motivation talks are more than one kind of legitimate bad news. I don't need a script to be upset by them, but sometimes scripts make me care more. Childhood is one big lesson that your purpose in life is to impress and entertain adults. It can be very hard to shake.