Learn Russian: Он переоценил свой опыт и провалил сложный проект. - He overestimated his experience and failed the complex project.

It adds the meaning of excess to the base verb оценить (to evaluate or estimate). Therefore, переоценить means 'to overestimate'. As you learned in this lesson, you could use the opposite prefix to say he underestimated it: недооценил.

Why does the sentence use свой instead of его for 'his'?

Свой is a reflexive possessive pronoun that always refers back to the subject of the clause, which here is Он (He). If the sentence said Он переоценил его опыт, it would mean that he overestimated someone else's experience.

I usually see 'fail' translated differently. How does провалил work here?

Провалил is the perfective past tense of проваливать. While it literally means 'to let something fall through', it is a standard, very common way to say you failed an exam, an interview, or a project. You might also recognize the related noun провал (a failure or flop).

Both опыт and проект are direct objects. Why don't their endings change?

They are actually in the accusative case because they are direct objects, but both are masculine inanimate nouns. For masculine inanimate nouns, the accusative form is identical to the nominative (dictionary) form, which is why their endings appear unchanged.