Reflexive pronouns are generally used when the subject and the object are the same person/thing. In your second example, "trust yourself" is the imperative form of a fairly standard sentence:
You should always trust yourself 🡒 Trust yourself
Your first example is superficially more convoluted, so let me start with a simpler example to establish the pattern:
You should not let yourself smoke so much 🡒 Don't let yourself smoke so much.
Going back to your first sentence:
You should not let yourself tell you differently 🡒 Don't let yourself tell you differently.
This sentence sounds odd to me, but nevertheless I think it's entirely grammatical. Yourself is the object of "let".
However, I also think you is fine:
Don't let anyone -- including you -- tell you differently.
Similar sentiment, but here the pronoun is part of the collective "anyone", rather than the object of the verb. It's an awkward phrasing, though, and might sound wrong to a lot of people. Personally, I would choose yourself.