I could sit here all day listing small things, but I think there are two major things I would have said.
Firstly, trust your gut (pardon the pun) instinct and stick to your guns. I wish I'd ventured into the world of chronic illness blogs and forums several months earlier - it would have armed me better for dealing with the medical profession and the general reluctance within it to accept anything that can't be fixed with one visit and one prescription.
Had I done so I think I would have had quite a bit more courage early on in standing my ground - I would have asked to be referred onwards rather than waiting for it to be mentioned, and I would have been far more assertive overall instead of allowing uninterested GPs to half convince me it was all in my head.
I have a very appropriate mug for this particular point:
(From Cafepress - I couldn't resist it!)
The second thing is more of a general life lesson but the learning came about during the diagnosis process. It's hard to explain without being terribly long winded, but a friend's saying of "You only want on your team the people who want to bat for your team" sums it up best I think.
In essence I suppose it's listening to the right people and disregarding the opinions of the wrong ones. I had several incredibly toxic "friendships" around me during the first year or so of being ill, and it took me a long time to wake up and smell the coffee in seeing them for what they were - damaging and unhealthy - and making the move to be rid of them.
Looking back with hindsight at all the hurt and general upset caused, I would honestly tell my old self to toughen up a bit and stop trying to make life easier for other people by tolerating people I don't want to tolerate. Even when I knew in the back of my mind that continuing on the path with some of those people was in fact affecting my health, I kept on so as not to upset the apple cart for others. I think it comes down to again trusting myself and my instincts a little more - I do have quite a knack for getting a bad gut feeling on first meeting someone which is inexplicable at the time but generally will come to light later.
Essentially, I think I'm a nice person. I don't like upsetting others or inciting confrontation. However, being ill has moved my boundaries and taught me new ones - it has shown me what I can't put up with and have no reason to attempt to do so. I've said before that in some ways I'm grateful for the effect on my life. I'm sure I'd have reached the same conclusions eventually, but I think sooner rather than later is definitely preferable.
Have I learned the lesson completely? No, but then life is all about learning and nothing worth having comes to you overnight.