"It's not a real disease" may or may not be true, but I would argue that is irrelevant.
Speaking as someone else who has been diagnosed with ADD, who is also unsure how it differs from simply being a lazy bastard, I can tell you that the drugs they give you for that are /very effective/ if you want to get more done.
They say it's not a silver bullet, but god damn, it sure feels that way. Unmedicated, I get one good day a month; I often get more done on that day than I do the rest of the month. Medicated, that day becomes somewhat average.
For me, most of the depression issues were caused by the fact that I was very obviously not performing up to potential, and when the stimulants fixed that problem, the depression issues mostly went away.
I actually was in a pretty similar situation around your age. Right out of highschool, I made a couple brilliant career moves, but after the .com crash I ended up stagnating from age 22 or so to 25 or so. At which point I started on medication. Four years later, I'm a published author, my company is paying my bills, and I'm generally not ashamed to be where I am. (Now, I'm not on medication anymore... long story... and I'm once again /much less productive/ so it's not like the problem is solved permanently. But medication can give you some very good years- years you can leverage to build skills so that your 'down time' is more productive, too.)