i am agitated and tensed and she tells me "relax!"

Your not over acting since agitation normally comes when your tensed. Of course there are people who don't show signs of nervousness but deep within they are and you can only tell if you hold their hands. Maybe the main reason why you feel agitated when others tell you to relax is because you don't know how to. Just take a few deep breaths and this will help you relax a little bit. When others tell you to relax, it means you are already showing signs that you're nervous and you can't just expect anyone to ignore you.

Sometimes while at work we certainly do not need comforting, as when it is very stressed sometimes we need to be left alone. I do not find you are overacting as at the moment of tension that to me would sound like mocking and not helping at all.

If you're already tense and someone starts yelling for you to relax, there's no way you're going to relax. They had to do relaxing things like speak calmly or help you by figuring out the source of your stress. Just yelling at someone to "RELAX!" never does anything to help, in my opinion. Trust me, I've tried doing that, too, and it never calms the other person down! It only builds and builds till both people are upset.

Overreaction actually depend on your own concept of reaction and interrelation. To some people reacting violently to hostility is overreaction. They would always try to resolve things peacefully. There are other people to whom killing men and destroying property are just natural reaction for survival. They do all these things without qualm. So you see, it actually depends on what YOU think and not on what OTHERS think. If you think it's overreaction and if you want to get rid of it, I think I can help. Let's first go to where the trouble has originated from. You are pissed off because you've found out that you actually look stupid when you get agitated. The office mate you are talking about proved more resilience by staying calm. It seems to you that he's actually enjoying your being in agitation. You don't like being asked to stay calm without having being told how to do it, and that is exactly what he does all the time. He asks you to relax, but never tells you how to. In a sense he always gives you half-hearted advice and that is what gives you the impression that he is actually enjoying your being in trouble. Get rid of your ego. You are agitated because it's you who is facing the situation. Getting stressed out in such situations is completely normal. You can get agitated as much as you want to. However you need to keep in mind that being agitated doesn't solve anything. Whereas if you could remain calm and relaxed you may go through every trouble with resolve. Show your officemate that you can handle things more calmly than he does. Prove to him that you are more composed and sane than what he thought you to be. Give enough evidence that proves that he has been thoroughly underestimating your mental strength.

Maybe you should listen to them. They are just probably trying to help you relax. But ofcourse it depends on how they tell it to you.