You have a girlfriend?!
Anyway, when I used a diaphram, I'd take it out after the 6 hours, and leave it out for at least a couple of hours. At that, I'd only put it back in if I knew I'd probably be having sex again.
Logistically, that generally meant putting it in during the evening, out in the morning, before going to work. I remember a few times forgeting if I'd taken it out, and having to check. Mentally, I just didn't like the idea of that thing being inside me, squeezing my cervix, any longer than it had to. I kept picturing in my mind the time my brother sucked the opening of a glass to his face, broke all the blood vessels, and having to walk around with this red ring around his face for a week or 2.
but....I digress.
You are aware that having sex doggie style is not recommended, don't you?
The angle can push the diaphram off the cervix.