Tsuihou no Kenja, Sekai wo Shiru: Osananajimi Yuusha no Atsuryoku kara Nigete Jiyuu ni Natta Ore

Haru, a lowly mage, works hard day in and day out under the abuse of his childhood friend, the great Hero Leticia. But one day, as he's enduring another of her scathing lectures, he decides he's had enough and quits on the spot. Now he embarks on the most difficult journey of them all—rediscovering himself! A fantasy story of overcoming abuse and rebuilding your life begins here!

(Source: K Manga)