I too am tall and over 50. Not quite as tall as you at 5'9", but I have a fairly substantial frame and usually wear a large 14 to 16. It has taken me some years to work out that small and dainty does NOT look right on me.
So here's what I've learnt so far:
Hair: After years of either short or shoulder length hair, I have finally grown it to just below my shoulders and will try to go a bit longer. I regularly wear it up with a sweeping fringe. My advice to you is a) go to a really good hairdressers and get a regular cut, keeping it as long as you can, maybe with a couple of layers in so that it doesn't just hang. Use a volume building shampoo, and don't overload with conditioner. I have hit the menopause, and passed the menopause. What about you? Could your hair condition be at all related to that? I take calcium tablets, and have done so for about a year. My nails are now so much better, and my hair is in a lot better condition too.
Stiletto heels look too spindly for me. A slightly bigger heel is better, and I am grateful for all those block heels that are now in style.
Bootcut, or palazzo trousers suit me better than slim fit or skinny, unless I am wearing a tunic top.
Dresses: I am not a natural dress wearer, but when I do, it looks better when dresses are just below the knee in a full or flared style. My shape (mostly pear) does not work well with slim fitting dresses.
Make up: I look better with makeup. If you're not really into make up, go and get a lesson at a make up counter or at a beauty salon and ask for a simple every day look. This will counter any fears you may have of looking bloke-ish by having shorter hair. Some jewellery will also help.
I have also started going to the gym (again) and started shaving off the weight, 100g by 100g. I have eliminated (mostly) wheat and dairy from my diet and this is starting to have an impact particularly from my puffed up tummy. I am hoping for a good size 14, as this weight loss thing doesn't get any easier the longer you leave it.......