"And to be honest, I don't even know why I let myself get down in the first place"
This is a quote from my favorite song (Sunflower by Rex Orange County.) It has been one quote that has stuck with me for many years, to the point where I may get it tattooed! I think it really speaks to how a part of our own happiness is linked to our thought processes and our attitude, rather than being completely due to the circumstances around us. I think people really underestimate the amount of control they have over their mindset via these thought processes. Many people get caught in patterns of negative thoughts, but over time, you can work to change those thought processes into something more positive.
(Note: this is not to say that everyone is in complete control over their thoughts, there are plenty of caveats to this idea, i.e. depression/mental illness, intrusive thoughts, OCD.)