It is forbidden to be sad? It is forbidden to be in despair?
It is normal for humans to be sad and sometimes even to be depressed. But it is not permissible to feel despair.
We have our trust in Allah and this is the only thing that keeps us going.
We believe that whatever happens to us, it is of our own doing and sins or because Allah wants to elevate our status in Paradise. Life is only a temporary test.
You have to always look at the full half of the cup. Failing to do so and only noticing the empty half of the cup would probably result in your depression.
When you are depressed and sad, you are the only one who is suffering. Your sadness will not change your situation or improve it.
This means that your happiness and sadness only affect you.
Try to feel positive and realize that Allah only chooses what is best for you. Allah loves to hear your voice urging Him and asking Him. There are hundreds of thousands who are worse than you but still, they are grateful.
Try to be one of them