I think what you've said makes absolute sense and you're not rambling at all...just talking out the thoughts in your head.
I can totally understand your fears but think that the plans you have made are appropriate given your problem with your thyroid and previous history.
IMO, I do think its worth pushing for an early scan, even if they can't see a heartbeat, they can still check that everything looks like its an order and appropriate for the stage you're at, can't they. I know a scan is scary for but its because it tells the truth and I think that's what you need.
Pains are normal, as are the tears, and in your case so are the fears. Take it easy tonight, drink lots and lots of fluids coz you'll need to replace what you've lost crying as well it being good for you anyway.
Wish I was better at sympathy but I'm kind of known for being a bit naff....my friends take the piss because of it - my words may not be eloquent but know that I don't think you're being irrational or stupid and would give you a hug if I could.