"My tortoise is so fast."
Translation:Tha an sligeanach agam cho luath.
The basic difference is that mo, do etc are used for what are called 'inalienables' - thing that are yours, only yours and permanently yours - mo bheul, mo mhathair etc, while the agam, agad construction is used for potentially more transient belongings. Of course there are exceptions and grey areas - do charaid may not be do charaid forever, and for some reason husbands and wives are often referred to with the agam construction - an duine agam, a' bhean agam. But that's language for you - infinitely fluid.
Because you don't love your tortoise enough?
I am unclear about the distinction between mo and the agam form, and whether it is as clear cut as all that. We are taught "mo charaid" but I have heard "an caraid agam" lots of times on TV. There may be subtleties here we are not picking up yet.