I'll take my turn to ask a few questions, and try to get a better idea of where you are having problems.
If you bought your gear at a garage sale, it may well be "vintage" gear, like more than 10 and as much as 30 years old. Beginners used to learn on longer narrower boards (many succeeded, but many gave up!) which have a lot of volume but not in the most useful place by modern standards. If you have an older long narrow board, balance will be most important! especially compared to a modern shorter wider board, and to a dinghy. It is also likely that an older rig will be considerably heavier than a modern one, which will add to your challenge. Older small sails were also designed for high winds, and usually for waves, making them less user friendly for a beginner (less control) and heavier (lots of reinforcement).
Sailiing a boat is different from sailing a windsurfer in some important ways. In a boat, the rig (and boat) will heel, or tilt away from the wind. And you have lines and pulleys to help harness the power of the wind. Steering is done with a rudder. We don't use any of that in windsurfing.
On the positive side, your sailing experience will help you with understand the wind, angles of sailing, where you can, and can't go (and why...).
From one perspective, windsurfing is a lot like waterskiing. The sail is your "power boat" and so as it fills with wind and powers up, you will need to lean back against the "pull" of the sail rather than letting the sail pull you over (the usual instinct). The power of the sail is transmitted to the board by way of the mast (as with a dinghy) but *also* via your body through the legs, again more like waterskiing.
Windsurfing is all about balance. When you uphaul the sail, your instinct is to PULL on it to get it up. And most people try to rush that part, grab at the boom too soon, and get pulled in, or get the rig up and pull it right in on top of themselves. I don't know if getting the rig vertical is where you are having trouble, but it is usually a big problem for beginners starting on their own. Your instincts tell you one thing, when what you need to be doing is just about the opposite! (Great sport, windsurfing... it challenges more than just your body!)
Bend your knees, keep your arms straight, and try to stand up as you lean back against the weight of the rig in the water, but slowly. As the water runs off the sail, it will very quickly become much lighter and you will need to balance against that change of weight, using your legs not your arms. Once the rig is out of the water, keep your knees and ankles loose, and your arms straight and learn to balance against the weight of the rig. You are now standing on an unstable board in the water and it will react to very small reactions in your body. A dinghy will not be this sensitive.
I hope I haven't covered ground (or water...) that you've already mastered, but we need a place from which to start in order to help you progress. Roger and I have been doing this for quite a while and have heard from many novices. One common difficulty is that as a novice, you don't know how to describe what is, or is not, happening, and often don't have the words to paint a good picture. So just talk about what you have for gear, what you do to start off, and what happens. And bear with us as we try to work it out in words!
cheers and welcome to the sport. The enjoyment is improved by the challenge to get to the reward!