A couple of thoughts:
1. If you have access to a boom you should try it. It adds stability so that you can feel the right balance you need to stay up on a single ski. Just a one or two sessions should be plenty. Once that muscle memory is there you can move to a short 5' rope off the boom and then try behind the boat. You'll be amazed at how quickly the boom can help you progress.
2. Keep at it. I've not been as big as you but not far off. I've probably skied at around 260-265 at my heaviest and I was able to get up, both feet in double boots, on a 71" Radar Senate. I've lost enough that I'm on a 69" Senate now, but just keep at it. Lose weight, keep exercising and keep skiing. It is worth it.
3. On getting up - if you are using a rear kicker keep your rear leg out when getting up. It is a lot easier this way. Arms straight, front knee between your arms. Let that front knee compress all the way to your chest between your arms and use the back leg like a rudder.
4. Get a pair a clincher-style gloves, the ones with the strap that helps you hang onto the rope. MasterLine, HO and Radar all make them (I prefer the MasterLines, but they all work). This will really help save your forearms and help your fingers get through the load that is required to get you up and out of the water.