Your goal of becoming a more confident and capable sailor is praiseworthy. I can think of a few thinks you might do short of enrolling in classes (though that is also a good way to improve).

1. The dory is a nice two-man sailboat, and there are lots of avid sailors who don't own their own boats and might love to sail with you. If you can find somebody like that, you can take turns controlling the boat. You'll learn a lot just by being out on the water with someone more experienced, and I'm confident that person will be flattered if you ask for tips while you are at the helm.

2. If you can sail in a bay where there are other dinghies on the water, try "racing" against them. Put yourself on the same tack and see what you can do to make up ground. Start by pointing just as high (or low) as they are pointing and putting your sail in similar trim. Then play around to see what (if anything) you can do to increase your boat speed.

3. Consider renting something like a Sailfish or Sunfish when conditions are warm and windy. The dory is a terrific sailboat, but I sure don't want to capsize mine -- though I've come mighty close! You can take on a lot of water in a hurry and waste too much sailing weather manning the bailing bucket and fending off lee shores. With a Sailfish or Sunfish capsizing is part of the fun. Those boats are perfect for taking chances!

Of course, a bit of light reading and playing around with sailing simulators can also help imprint the theory behind it all.

If you can safely raise the sail in windy conditions, safely strike it, and avoid letting the mainsheet get cleated or snagged, you are probably ready to take on anything that protected waters can throw at you. 

���I don't know if you've seen it, but Spinsheet has crew parties early each season to match boat skippers and crew. There's a wide range, but racing is on the mind of a lot of the skippers. Some crew will be intense, others not. It might be something to check. I race Wed night's on the Magothy and I'm the youngster at 57. Ours is a great bunch and I learn things even after owning my own 30 footer for years. Not dinghies, but still good eexperience. My skerry is very different from keelboats but we have bull sessions on the big boat and I can pick brains. Look at https://www.spinsheet.com/crew-parties

   You are certainly on your way to being an experienced sailor. Tiller time, tiller time, and more tiller time makes an experienced sailor. Sure find a class, but keep getting out there in you yacht. She will teach you the best.

You say you only run aground a couple of times per trip. That is not sailing skill, but navigating skill. "Off Center Harbor" has some good videos on navigating. 

Keep having fun,

Joel