I would also have suggested not to say anything about MH because it is always likely to be judged and you don't want your current state to be destroyed just because of issues of the past (since you know that you can handle things better now). That you need flexibility can be said without having to say about past, you can just say that you are a caregiver so at times you need time off or something.

but if you don't really wanna lie at all then do say a bit about past MH but sound like you are confident enough now not to repeat those in case of your present pets. And state clearly that now you know you can handle stuffs.