Help me leveling? (xbox one and borderlands 1) pls

does someone wanna help me leveling cause i am lvl 44 and there isnt anything left to make xp’s anymore and i read if you do the story you will have something with 50 level but i only had at that tiome something under 40 and after that i done some general knoxx missions with a friend and now i am level 44… how can that be that i have such a ■■■■ level?