How to be happy as an expat in Mexico

Well Cheryl,
it's a matter of being willing to let go of fixed ideas and be flexible.
People here are family oriented, time is relative, eating habits are different, rules from other countries often do not apply.

Expats have to understand these things or they won't be happy at all . There is a tendency toward moving their world from their country to another and that is not reality.  You need to work within the structure and rules of the country you visit. I've actually seen a lot of that traveling over the years. People bringing all of their things somewhere else and expecting it to be like the home they left.

Happiness is a state of mind that comes from some place within us.  If one expects that a change of  external scenery will ultimately bring lasting happiness, that person will eventually be disappointed even if they relocate to a paradise. 
That said, I love living in México.  México had been a vacation retreat for decades before retirement and moving here full time.
For the last four years, I have lived in a remote, ocean front Villa on the Caribbean.  Many people tell me this is paradise. 
There are not a lot of people here nor are there convenient first world, retail services that I enjoyed most of my life in California.  The truth is, I would be frustrated if I expected a Trader Joe’s, a Whole Foods, or even reliable Amazon delivery.  As a 6’2” man with American size 12  feet, I can’t find any quality clothes or even flip flops that are big enough even in the closest city. I am only mentioning a few of the many potential frustrations one will experience.  For better or for worse, México is changing and Fellow readers, please don’t waste time commenting on this detail or that.  The point is that, even in paradise there are aspects of life that can make us unhappy if that’s what we choose to focus on. 
I love living in México.  I love my life.  I have learned to “accept those things I cannot change” while I change those few, little things I can.  I was happy before I retired to Mexico but I knew that I would not be financially able to retire comfortably in California.  I choose to accept a life without all the first world trappings for a beautiful world where I am even happier.

Maybe the movie that I'm suggesting everybody watch... would actually scare people away from wanting to immigrate to new planets, worlds, and nations... but watch Dancing With Wolves which stars Kevin Costner and remember the last half of the movie when he almost couldn't even speak English anymore and the Americans thought he was a Native, not an American (for those who don't know, many Natives of the continents now known as the Americas are offended to be called Americans and prefer their Native names such as Cherokee, Navajo, Appache, Guaraní, Wichí, Mapuche, Inuit, Métis, etc, and the land wasn't called America). If we go to an alien planet thinking we're not going to eventually become alien ourselves... we should first realize the fact that the average person begins to lose their own language after only 6 months of not speaking it!!! Most people continue speaking their own language so they don't face that dilemma but it goes to show that it doesn't take much, and doesn't take a lot of time, before you assimilate a new culture and completely forget your old one. Of course you'll retain some things. The point is, a person shouldn't move to another country if his/her goal is to hold onto their previous one. And so for those who want to hold onto their previous country/culture, etc, and are FORCED out... by war, political upheaval, health needs, etc, that's truly a tragedy. And for those who are excited to learn a new way of life, kudos!