I think that we have similar lifestyles Lisa. About 95% casual - and the rest is just a notch up from that with the occasional dressy function thrown in maybe once or twice a year! However, I live in a much different climate than what you do and that will definitely affect your choice of essentials as compared to mine. I also remember that you admitted on another thread awhile back that you tend to buy clothing items without trying them on at the store first. (I used to do the same thing.) I realized though that that was really contributing to the chaos in my wardrobe because I would often end up with things that weren't a perfect fit but not bad enough to return and so then I'd keep them, but more than likely would not ever pull them out to wear because I didn't feel all that great when wearing them. Once, I started mindfully shopping rather than mindlessly shopping my stress level about my wardrobe significantly decreased - so, I'd recommend that one thing you try is to change the way that you shop. No more breezing through a store and just picking up this and that - thinking oh, if it doesn't work I'll just return it! It made a huge difference for me - so it might help you too.
My essentials (off the top of my head) include: (for the current season)
1) Jeans - BFs and Slouchy Skinnies (black, charcoal, blue denim, ivory)
2) Pull-over V-Neck or Crew-Neck Knit Tops (gray, medium blue, ivory, navy, pink)
3) Button down blouses/shirts - silk, cotton (gray, medium blue, pink, ivory, navy)
4) Ankle booties (black or navy or charcoal gray and one in a lighter color - taupe)
5) Sneakers - white, black
6) Moto Jackets - Black leather, Gray mixed media, Navy/Black mixed media)
7) Blazer - Gray, Black
Other items that would not be considered essentials but that I feel like I need to have in my closet at any given time are:
LBD - or an equivalent to the LBD
Dressy shoes - pumps, ankle strap pointy-toes, etc. (black)
Tube skirt or Pencil skirt that can be dressed up or down
Trench Coat
Wool Coat
Dressy Black or Navy or Charcoal Gray Trousers
Casual Dress
I have really narrowed down my color choices and so that has helped me to curb my haphazard shopping habits. I chose 3 dark neutrals - black, gray, navy - and 1 light neutral - ivory. Then I chose 5 to 6 main colors - lighter shades of gray, navy, medium blue, pink, ivory, and various shades of berry. Then I chose a couple of color "pops." (These included some colors that I love (citron) but cannot wear a whole bunch of) as well as some others that I just like, but didn't want to expand my wardrobe to include them in my main colors - such as teal and other cool shades of green. Scarfs, handbags, jewelry, etc. will often times be in one of my color "pops." I have also added cognac (a really horrible color for me when next to my face!) in the form of a color "pop."
Organizing my wardrobe this way has made it seem as though the items seamlessly mix and match with each other - for the most part they do!
Whatever you do - go slow. Take your time - and know that just because you've decided that you're going to change your strategy and get your wardrobe under control and workable, that you're still going to make some mistakes - so go easy on yourself! Make it fun - not a burden!