There is a general expectancy, when someone comes in to work, that their personal issues are left at home. This seems harsh in cases of extreme loss or difficulty, but it is a good rule for the otherwise endless list of life's little complications that could negatively influence a business.

Right or wrong, even if you do not feel happy, you are expected to "show an effort" in getting better over time. It was very unfair of your boss to expect that timeline to be one day, but any good manager would have to agree that, eventually, they want you feeling better.

Like other answers have mentioned, refusing to share just how big your loss is means that your boss can do nothing to help you. She may assume that you are allowing your personal issue to affect your work performance, even though that may not be true.

I know you are looking for a new job, so this may not be applicable to your situation: in my workplace, if I wasn't comfortable talking to my boss or if she just seemed unfair, I could talk to HR about a difficulty such as this, and they may "persuade" my boss to give me some slack. That could harm your relationship with your superior, or she may just realize the issue is bigger than she thinks and back off. It really depends on the type of person she is and the corporate culture of your employer. (I am located in the U.S. - so there may also be a difference there)

As far as the on-topic answer goes: It would be harder to get hired as someone who is humorless and depressed than if you pretended to be okay. A completely new company may think you just hate your job. You could explain the situation, and they may be more understanding - but if there is another candidate with just as good of qualifications then they are likely to choose the "less risky" option.

If you have great qualifications, they may take a shot at hiring you anyway, as long as they believe you'll stick around.

For those days where you just can't pretend, you seem to be aware of the instances where your co-workers are expecting certain responses from you. Some will feel negatively when you don't act like your usual self, that isn't really your problem in my opinion. You can't be forced or expected to act a certain way (emotionally) to make someone else happy, just like they can't be forced or expected to act a certain way to make you feel better. Only you can make yourself feel better, but while it is necessary for you to take care of yourself first, make sure you are not taking your pain of loss out on your co-workers - it is not their fault and some may just be trying to cheer you up.

Now the off-topic answer: I am not a psychologist, but I have had my own difficulties - recently - that sound similar to yours, and I didn't realize that I was causing myself most of the hurt I was feeling. I am not saying this applies to you, but I put it here in case anyone in the future happens by it as well.

Sadness and pain are both very real and powerful emotions. Dwelling in pain and sadness, however, (usually called depression) is sometimes a chosen action, even when its because we just don't know any other way to respond to it.

"..but why would I choose to feel this way?"

Like I said above, it could be you just don't know any other way to respond. In reality, I don't know, because it can be different for everybody. That's a question you would have to ask yourself, and, if necessary, with outside professional help.

For myself, a process called "Inner Bonding" helped immensely and nearly immediately, but I'm sure its not the only path one could take. There are numerous strategies out there for dealing with our negative emotions, and many of them are free to look up.

If you are tired of feeling like you do and feel "stuck", its worth looking into.

This is a link to the free pdf.. I really recommend reading through it. You'll know if it will help or not pretty quickly I think. It starts with a relationship example, but read a page or two at least.