I realized a while ago that it's possible to simply choose not to be stressed. So, when I have several things to do in a short amount of time, I sit down for a minute and plan, rather than get nervous about whether or not I can complete every task in time.

Even if I'm not able to, there is still a best possible solution that exists, even if the solution does not involve completing each task on time.

Is it physically possible to complete each task except for one? Then do it. Do you have some time to spare afterwards? Looks like you can now do task number fifeteen.

I always complete the tasks that can be completed the fastest first, so I will have more done. I always look for ways to kill two birds with one stone, ways to make time (doing homework on the bus or before classes start, for example so that you can use other time to either relax or do the rest of the tasks), ways around a certain problem, ways to make the process of fixing things as quick as possible.

Do not ever chose to be stressed instead of fixing a problem. Important thing needed to complete a task breaks unexpectedly? Is there time? Hurry up and get a new one. Is there an obstacle that can't be removed? Hurry up and change your plans. Is something unavoidable going to cause a major inconvenience? Well, that's that. Don't let it stop your productivity completely. You must chose not to care.