If you invite her here, I promise to be supportive and sympathetic, as I have been (to the best of my recollection) to all the SOs who have posted here. This is the section intended specifically for us; if you're looking for similar experiences from trans people, you may want to try posting in those sections. I don't know how many SOs will have had (or want to share) this experience from the other side... I know I wouldn't even if I could, since it'd seem cruel to repost nasty comments I'd said to my wife so that they could hurt you too*.
As for "since you let yourself go, why can't I transition," yeah, I can see why that might not be the most persuasive argument for her.
*I haven't actually said anything like this to her, but hypothetically.