poor you, you sound like you are having a really tough time,
september is a long time away and a lot can change in the sense of sleeping patterns,
if you love the idea(which you don't need to commit to now by the way), it sounds like a great idea, there is no reason to feel guilty about it, you would be leaving your baby with the woman that successfully raised the man you love, so well qualified to look after a nine month old, it's only a western thing to not hand over babies to other family members, they will both get a lot out of it,
your children won't suffer, they deserve a well rested happy mummy, so anything that can help is wonderful,
is there anyway you can have some time out before then, you sound like you need a chance to recharge, it can be a vicious circle, sometimes just little breaks can really help you get your equilibrium back
it's very common to feel like this and sounds like you are not sticking your head in the sand about how you feel,
and the slow time thing, I promise you, one day soon this will all feel like a very long time ago and you will wonder where the time went, I know it doesn't seem like it now, but it will get better, and you will look back at this and wonder how you felt like this, grasp any help offered, tell your intuitive MiL yes please, any help would be wonderful, and don't you dare feel guilty.
and keep talking about it, as it stops build up.