"All you have to do every day is to be better than you've been yesterday. Your worst enemy is you yourself."
This is so very true, learn from mistakes, adjust in life circumstances, look back and review in order to educate and better yourself. Don't live in the past as you're not going that way but also don't ignore it as it has ways of teaching us, just don't live in those moments.
Too many try compete, judge or compare their life to other's lives, even if another person seems like they have it all, often the underneath you don't see, is things that aren't so 'perfect'. So if you are focusing on your own life, your achievements, your past to learn from and keep pushing forward to be a better person today than you where yesterday, then you've succeeded.
Same as one's happiness is not based on circumstances, but one's ability to experience and accept current circumstances they are experiencing. Yes things could be better, often this is the case, though doesn't mean you can't be happy while in a bad or less than ideal circumstance, while you find ways to change those circumstances to make life better.