Borderlands 2 - Completed Game With Siren But Still Lots To Do

Yesterday I spent most of my day playing Borderland 2. I am currently using a Siren and she is a level 33. I have completed over 80 missions total and from what I am hearing there is probably more. I may have completed story mode missions and side missions but I am still finding it hard to level up. I thought by doing all the side missions first and work on story mode missions as a second priority I thought I would get leveled up faster, but that simply wasn't the case. 
I have about 6 undiscovered areas still left to find. I have only found 2 borderlands symbols that are on hidden throughout the map usually found on a wall either inside a building, cave or outside on a rock. They are hidden well. 
I have managed to sell by rare loot relic by mistake and I am noticing that I am finding way less rare loot with out it! This is a relic you don't want to sell so put it in the Bank if you don't wanna use for a bit to avoid selling by mistake. 
I am going to continue leveling up my Siren Character until I reach a level 50 then I will start a new character. 
I have unlocked "True Vault Hunter Mode" and hope by doing so I will find better loot and get leveled up faster now.