Believe that life is worth living and enjoy and your belief will help create the fact.
Live every day on a fresh new start. Don't be held back by what happened yesterday, the day before, the week before, the year before, and so on.
Be true to who you are. Stop trying to please other people or be someone else. It's better to be an original version of yourself than an exact duplicate of someone else.
Quit complaining. Don't be like the howling dog, always howling and never doing anything. Stop complaining about your problems and work on them instead.
Be proactive. Stop waiting for others around you to do something and take action yourself instead.
Rather than think “what if”, think “next time”. Don't think about things you can't change (namely what has happened and thoughts of other people) or unhappy things because these are disempowering. Instead focus on the things you can action upon. That's the most constructive thing you can do in any situation.