I've been dealing with negative thoughts for the past few months, and I'm finally understanding when, where and why it all started which helps me deal with it. Although it's tough at times, I don't see it as something "sad" but rather as an opportunity to learn important lessons that I can add to my luggage of life lessons. :)
If you're dealing with something similar where it's affecting your mental health, please speak with someone about it - either a friend or a professional. You are NEVER alone.