Don't completely dismiss "what if" thinking, but curb it. Before you act, it's a good idea to weigh, "If I do this, what will happen?" That prevents people from making mistakes they can't undo. But only to the extent that you have considered the consequences of actions or speech. Beyond that, it is counter-productive.
You have much more power than you think you do -- just with positive thinking, you can create a good day today, no matter how the morning starts. It's important that you do not let the way you feel when you wake up dictate how you feel throughout your day. The day's events are not set in stone. As each minute ticks by, you can take it and make it what you want it to be.
Every morning, at the end of my shower, I quietly say this ritual:
As I turn off the shower, "Thank you shower."
As I put my hands on my shoulders to start pushing off the excess water, "Thank you fire and cold".
As I brush off excess water before drying, "...water".
As I take a deep breath, "...air".
As I grab my towel just to wipe the excess water off my eyes, I feel my feet on the ground and say, "...earth..."
As I grab my body oil that I will rub on before drying, "and spirit."
Then as I take my towel to start drying, "Thank you Mom and Dad for my life.
Thank you that I can wake up, that I can breathe. Thank you that I can smell and taste, see and comprehend, hear, speak, balance, feel and heal. Thank you for my home and for my family and friends."
Then I dry and get out.
This may sound excessive, but it establishes a positive attitude first thing in the morning. It's easy to say "be thankful..." but if you don't feel it, then it doesn't work.
Avoid saying I, me and my when you speak. Listen to the other person and don't offer your opinion unless they ask for it. Do not complain.
It takes two to argue. Never argue or try to verbally defend yourself. Be agreeable even if you're right. You can defuse arguments by not participating.
When you become angry, chill out until you can relax again. Try.
Then realize that everyone is different, they all make mistakes, do what you can to resolve the issue without become problematic yourself, and move on.
If you have a depressive episode that "paralyzes" you, do something. Don't let your feeling stop you in your tracks. Do anything: laundry, moving things, cook, walk, just do something. You have to move.
Also, remember the Serenity Prayer: My Dad used to say this a lot, and he lived it. You need to know when to let things go, and when to act on them:
God, grant me the serenity to accept the things I cannot change,
Courage to change the things I can,
And wisdom to know the difference.