I'm so stressed out today.You mean you are very tired.so you can say 累 今天我很累。 今天（我）累死啦（very spoken） If you mean you're annoyed by somebody or something today,you should say 烦 今天我很烦。 今天(我)烦死啦。 Today is so stressful.you cannot translate this directly,because when we express our feeling,we often use person as subject.This sentence seems to be similar to the first one,if you want to say today you have much pressure,you can say 压力大 今天（我）压力很大。

I'm not sure but you could say 今天压力太大 -- I think that actually means something like "the pressure is too high today" so it's not exactly the same thing. And you could also specify if it's 工作压力，考试压力 or whatever. Or I think I've heard people say something like 今天的安排很紧张 but again, that is more like "My schedule is really tight today." It's a great question -- I'd love to know if there are better ways to say it!

今天我感觉压力很大。今年我感到非常疲惫。今天我感觉很累。今天我快累死了。字面翻译第一个比较好。但是第二句更能表达stress out，exhausted的感觉而且比较书面语一些。第三句和第二句差不多。最后一句比较口语化。意思都相近。

stress out 在中文中表达“因为过度紧张或压力过大而感到劳累”

great. that's what I wanted to know. thank you.