What should one say to themselves on a daily basis as a mantra, etc. In order to fell positive?

Either "Boy, I'm glad I wasn't in Las Vegas that night" or "I'm glad I'm not in New Orleans for that storm" etc.

OR

It's a sunny day, I'm glad I have a job,
my car runs well, I glad I have a.c. In my residence, etc.

OR

I'll be happy when I get that Ford F250 truck

I can't wait till Friday and quitting time.

I'll be happy when I'm retired. Etc.

Neither. You don't want to set your emotional well-being up on the basis of not being unfortunate, or on some day being more fortunate. You want to base it on what's good right now.

Yesterday I read an article that notes that people who write ten things each day that are good about their lives that day for two weeks, no skipped days, tend to improve their outlook on life. Even if the only things they can write some days are "I didn't have to go to the hospital after all" or "my lawyer says it'll probably just be a fine and not jail time" they still had a better outlook.