Good that you have learnt from this event.
The key thing is not to panic. When something goes wrong or unexpected, unless you're on fire or about to crash into terrain, the first thing to do is sit on your hands and think about what you're going to do.
With a door still on the main latch, you would of had plenty of time to get the POH out, and refer to it for guidance on how to get the door closed.
There was no need to open the main latch at all, the top latch on a tommy can be undone and done up completely independently from the main latch. The metal loop often gets a little bent out of shape so doesn't fully latch both doors without a bit of wiggling.