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A good way to balance fights, is to change between fighting on the offence and fighting on the defense. When you're fighting offence, give the other person a chance to react and block or dodge your incoming attack. This way, if they get hit by you repeatedly, it's their own fault for not having their character avoid your attacks. When fighting on the defense, take one hit for every two you dodge/block. Every hit you take also has to wear down your character a little. If it's a physical fight, a blow to the arm makes that arm weaker, and more difficult to use. Your character will cringe now when they use their arm that's been hit and favor their good arm. If it's a fight with super powers or magic or something, say the enemy has the power of electricity, a bolt they shoot out grazes accross your side, leaving a burn that streches across your hip, making it painful to stand on that leg, so your character holds most of their weight on their opposing leg. This makes it so they can only dodge quickly in one direction, and their injured side becomes their weakness.
Also, another thing to remember is blood loss. If your character gets a reasonable sized cut, or a wound that bleeds heavily, they become woozy and weak after a short while due to blood loss. This is another good injury to give your character.
As for skulking off, do your best to always be with at least one other person. That's all the advice I can give you there. Sometimes if you have a character who would do something like that, it's better to have them do nothing instead of leaving. Or try to get involved in whatever the distraction is.

Hope this helps.

I had the same problem when I began roleplaying, and although I still have a tendancy to make my charecters over-powered, I usually incorporate several weaknesses that I know people will exploit to make up for it.

In one of my roleplays a telekinetic charecter of mine has a large physical weakness to light of any kind. When in pain she cant focus to use her powers, making her lean heavily upon others, where that came into play was this: A group of soldiers burst into the library that my charecter and other survivors were hiding, she began to throw heavy object at them with her mind, and the soldiers threw a flash-bang, a grenade that produces light, it burned her skin intensly and she fell to the ground, the others had to fight them off (she had only succeeded in killing one, which is what I did to keep the power level under check)

After a little bit moderating yourself becomes like second nature, its really easy to get carried away but just remember that you are always able to stop yourself.