I agree with Nihlum, especially in regards to changing mentality. The OPness of a character is less about "what can they do" and more about "what are they doing for the game and/or story?"
In a story, an OP protagonist needs to be relatable or likable, even without their powers. An antagonist character can absolutely be dispizable and powerful, but ultimately, that kind of villain only represent the physical aspect of his struggles and a direction to go. A force of nature. You would need a second villain with more pitiable or redeeming qualitys to go with along that.
For a game, your character needs to help make the game fun. If your just a shmuck, do what you want, but if your character is ANYTHING above the other players, you need figure out what your roll in the "story" is and to use your power or skill to enhance everyone else's experience, not your own. With great power comes great responsibility, right?
As an exercise, here are the kinds of character I would challenge you to play:
You. Play what you know, a character that shares your problems, most of your experiences, and your deeper thoughts. Either way, this is a good way to not only make a believable character, but also to vent your frustration and express your self. BUT the catch is you can't give him/her any powers that would eliminate your inner struggles.
A person you know. Find a person who's personality and background you think understand. Or else, pick someone you dont understand and really try to figure out how and why they act the way they do. This will help you learn how make characters more believable and should give you a better insight into your friends, family, enemy's, and people in general. ( feel free to exaggerate their personalitys, but careful not to give them powers that would make them deviate from said persona)
A person who is better than you. This kind of character can be as kick@$$ and godlike as you wish, but he/she must have at least one or two admirable or virtuous personality qualities that you deeply wish you had, such as wisdom, bravery, kindness, patience, good people skills, etc, etc. Essentially, a Hero. I'm not going to lie, your going to fail time and time again to follow that trait, (and possibly piss off a lot of other players in the process). But keep recycling that character, and as you learn how heroic people think and act, you will not only have a character that others players will love, but you will have taken on those honorable qualitys, your self.