Penalized for booking typo and must purchase new ticket!$$$$$$!

I booked a RT ticket LA/AMS via LHR for a 5 day trip. Accidentally clicked April instead of March for return flight and didn't notice until arrival in Europe (I'd been focussed on my departure,) and brought it to the attention of the airline immediately. I was told to take it up with KLM when I arrived in Amsterdam. I've spoken to representatives both online and off, and have been told that it is impossible to fix my ticket, and I must purchase a new one to go home. I can understand their skepticism, if I appeared to be adding a few days onto a vacation, but 5 weeks??? I am an executive in a position with high responsibility. After recognizing my error, I did my part by addressing it right away, yet I've received no help at all.
Hard to believe that the technology is not sophisticated enough to make an adjustment, and that I am being penalized for the slip of a finger. Any ideas?