What an awfully difficult decision. And what a sage oracle of a mum you have!
I truly don't know what I would do in your shoes, I'd be exactly as torn as you are, but I didn't want to read and not reply.
You've identified that your motivation for using the embryo is potential future regret. Here's some more questions that might help:
How would you feel if you received a letter from the clinic tomorrow saying that the embryo had been accidentally damaged, thus unusable?
How would you feel if you had the transfer and it failed?
How would you feel if you had the transfer and it succeeded?
If you didn't use the embryo and five years down the line you got ill and died (god, sorry to be so morbid!), would it trouble you that you had not used the embryo?
How would you feel if you decided not to use the embryo, and then in a few years, you started to get broody and wished you had another child but it was too late - would you be able to 'forgive' yourself and move on positively for the child you have, or is it something that would cloud every day?
I think this last question is important - it's your 'worse-case scenario' so should be examined and confronted now (I did some press skills training and the trainer said the best way to prepare for an interview is to identify and prepare an answer to the worse possible question the interviewer could ask you!). To have another child, I would need to start fertility treatment so I'm not a hundred miles from your situation - I have the same 'worse-case-scenario as you i.e. potential regret at not trying.
I have come to peace with my decision (not to have another) through being 100% confident that, if it did come to the worse-case-scenario stage when I 'regretted' not having tried for a second, I would be gentle and 'forgive' my former self, knowing that every decision made at every step of the way was made thoughtfully, and with the best interests of my family at that moment in time. I honestly believe that it is impossible for a past decision to be a 'wrong' decision, if it was made cognisantly and with care, based on the information available/the situation at the time - sadly we can't make our decisions with the benefit of hindsight.
Anyway, that's the place I have reached, but it's very personal, and it might not be a place you can see yourself at yet (or ever...)