If ...... I would not lose temper.

If I were you is the correct usage because it follows the rule of the subjunctive mood. The subjunctive mood is used when talking about hypothetical or imaginary situations, and "If I were" is a standard phrase for expressing unreal or contrary-to-fact conditions. "If I were you" means imagining oneself in the other person's position. The other options, such as "I was you" and "I am you", do not convey the hypothetical meaning, and "I am not you" is incorrect in this context.