I always thought of Thor as always worrying about his worthiness and amplified so after the death of his father. I think he's worried about living up to the gap his father left, which is a big pair of shoes to fill.
When he gets fat, he's clearly depressed and decides to forget his responsibilities by eating, drinking booze and playing video games. He sarcastically jokes about things and doesn't take things seriously and is afraid of taking up the responsibility again of being a hero because he doesn't believe in himself anymore. When he meets his mother and realises that he's still worthy, it let's him now that he's almost been forgiven for the time he's wasted away, and that he can redeem himself by helping un-snap everyone and later fighting Thanos again (Something he would of most likely run away from had he not had the pep talk, and use his re-found courage to face the nightmare that was plaguing him for years).
I think he gave up the mantle of King to Valkyrie so that he can seek out the things he's ignored, like abandoning Jane and fix their relationship, protecting the nine realms via diplomacy and peacekeeping, and perhaps whatever else Asgard was supposed to be doing (Like protecting the dwarves that forge weapons).