I really don't blame you.
Honestly, I wonder if the whole download system was forced on Asobo by Microsoft for political reasons (getting everyone on an Xbox Live account), because it's so flaky and badly implemented it's not really in keeping with the quality of the rest of the sim.
Good news here is that the download has finished and it
does appear to be working again. Not tried a flight though (just downloading the three world updates). But at least it's let me back into the sim. Fingers crossed.