Hi folks, this is my first post here.

I've been doing web/graphic design for almost two decades now, which naturally really helps with getting into ebook creation, but even still this latter is all very new to me -- and I find myself feeling so very confused and discouraged by what certainly
seems to be the complexities of it all (far worse than web design, in fact!).
I managed to create my first epub using Sigil, doing all my coding "by hand" and everything validates perfectly with no errors or anything. Unfortunately, the only "real" ebook reader that I have to check my work is my iPad, although I do have the Kindle previewer, too (which sucks). Even just on the iPad, though, I first managed to get my epub looking
great in iBooks, but then I downloaded the Kobo reader and my wonderful design just falls apart (so to speak), with my nice full-page images getting broken in half across separate pages, and other weird things going on.
Is this normal, just the way things are, that basically it's just not possible to do
nice designs not only for a particular platform (like iPad) but that will work nicely on any ebook reader, and in any software that one might use (for example, in both iBooks and Kobo)?
Or, when all you pros out there design your epubs, do you pretty much design things to look nice one particular platform and in a certain software, and to heck with the rest?
I feel so very discouraged, and I've barely gotten started -- surely this issue has come up before? I've looked all through the various forums here but haven't been able to find any discussion on this. ANY thoughts/suggestions would be much appreciated!
(PS. Hope I've posted this in the correct section of this forum!)