It's been a long time since I've played Ars Magica, but if memory serves the game is meant to be played with everyone having more than one character - a wizard and one or more non-magical sorts who assist with the day-to-day affairs (bodyguards, stewards, etc.). Typically only one or two players per story arc play wizards while the other players run support, then the focus changes to give the support players center stage in the next season.

Think of it like your favorite episodes of (series with an ensemble cast). Each episode focuses on just a couple of main characters, and may or may not involve a greater storyline for the season, but the other characters are still in the background doing things.

If this trading off is not happening in your group, you are well within your rights to say that you are not having much fun playing the heavy while everyone else gets to play with magical toys, and ask to be allowed a character who is of a more equitable power level with the others. The tank can just as easily be a player-controlled NPC who just comes out to make a witty observation and defend the group from time to time, then melts into the background again.

Now, if you just don't find the game system or the setting in general fun, there's not much to be done about that. You can either play along just to be sociable and wait for another game to come along (as they always do), prepare a game of your own to run in a system you enjoy more, or wish everyone fun with their game (in a completely non-sarcastic way) and go do something you enjoy more until they are doing something you feel you would enjoy participating in.