I would not say “not believe too much in your efforts”, I think the tendency to simply scale down enthusiasm can be toxic in its own way.
I like to remind myself of how Václav Havel said it:
Hope is not the conviction that something will turn out well but the certainty that something makes sense, regardless of how it turns out.
Yes, being enthusiastic about false goals can lead to devastating results. Being hopeful by realizing that your work does make sense even if you won’t necessarily see results of it, that’s much more sustainable source of motivation.
Also, remember that no matter how it turns out you will learn something on the path. If anything, this is one of the “certain” parts.