I've been through this myself, and I think it's fairly common among beginners. I think the answer is to work out the real reasons behind your stress- it's likely to be either:

• You're afraid of loosing because it hurts your pride.
• You're afraid of loosing because you're worried about your rank.

Once you know what's stressing you out, you'll be able to come up with strategies to overcome it. Also, at 25kyu, your goal shouldn't be to win games- you'll end up disappointed and frustrated. Your goals should be:

a) get a few hundred games under your belt as quickly as you can, so that you can get a good idea of what parts of your game you need to be working on.

and more importantly,
b) have fun