We all have bad days, and that is okay; having bad days are normal, they're hardly something we can control. No one should make you feel bad for venting or talking about your bad days.
Once in high school, someone asked me how my day had been. I answered them and told them that I had had a bad day. When they asked me why, I continued to tell them about the reasons my day had turned out so bad.
And what was there reply?
How could you be so inconsiderate? How could you complain about such miniscule problems when people are dying and starving and suffering every moment of every day?
And I wanted to tell them that just because other people have had worse days than mine, that doesn’t mean I can’t be upset. Just because some people suffer a great amount, it doesn’t mean others can’t still suffer.
If you have a bad day and need to vent about the things that made you upset that day, it does not mean that you are a bad person. It does not mean that you aren't grateful for the things you do have. It does not mean that you aren't sorry for the people who are suffering more than you. If you have a bad day and need to vent about the things that made you upset, it means that you are normal; you are human.
Telling someone that they can't be sad because someone else has it worse is comparable to telling someone who's happy that they shouldn't be happy because someone else has it better. You are allowed to be sad or angry or upset. You are allowed to be happy or excited or feel accomplished. You are entitled to whatever feelings you feel, no matter what else is going on in the world.