"To bear", in the sense of tolerate (literally, "carry"). "To bare" means to expose or make naked. Yes, you can use endure and bear as synonyms. The nuance is that they have different origins, and that tells us the emotion/formality of each word.

You can definitely use 'endure' and 'bear' interchangeably in this example.

Not really. When you bear something it's (usually) focused more on the subject performing an action which is difficult (such as bearing a burden) while enduring is moreso focused on an action afflicting the subject (such as having to endure criticism). If things were switched "Its one of the hardest things I've ever had to bear." would give the feeling that the action was the subject's choice or at some point could have halted their struggle.