Sorry to hear about your bad experience and the rude staff. All that aside, it sounds like this problem originated at MCO.
When you say "ticket" do you mean a boarding pass, or an e-ticket receipt? The latter is what would have confirmed the change in your itinerary from MCO-ATL-DCA to just ATL-DCA.
If you were never given such a receipt and you never got one by email, then it's likely that nobody actually changed your itinerary, and it was auto-cancelled as soon as the MCO-ATL flight closed without you onboard. Unfavorable as this practice might seem, it is an airline industry standard.