Don't look back. Yes, I am talking to you.
So, what are you sad about?
Is it a heartbreak or rejection?
Your best friend getting along with others?
Financial issues or stagnancy at work place?
This all is okay but Let's discuss this first...
Do you remember you had a hobby?
When was the last time you did something you love?
Last time you smiled at your own reflection?
Going out to a park and breathing in some freedom?
Parties with teammates go on, but do you remember when you and your family sat together talking about random stuff?
When was the last time you sat alone smiling after a day spent doing all this you love?
Don't waste your time crying on your problems, they are not going anywhere. First go do what you love, at least 10 years down the line you will not regret much.