I was you a couple of months ago and I found the idea of my mum and DH not being around very stressful and overwhelming, but the good news is it's not as bad as you think it's going to be!
The bit I found (and still find) most challenging was the morning routine. DH leaves v early and then I'm left to get the kids ready and the oldest to nursery. I get up 15 mins earlier than I thought I'd need to so that I can either have a quick shower to wake me up (if the baby is asleep) or feed the baby back to sleep so that I can have a shower afterwards. I find this is the best time to get the shower in (while DS1 is still asleep too) with no interruptions and makes me feel a lot better, especially if I haven't had enough sleep. I prep as much as possible the night before (DS1s packed lunch, bowls and cups out ready for brekkie, etc) so that it's easy to get things done in the morning, especially if I only have one hand while carrying baby.
Baby then usually falls asleep in the car on the way to nursery, and most mornings will have his long nap then (2-3 hours) so when I get back home I do any urgent/important chores and then if I'm still tired I'll have a lie down and try to nap for a bit (important to fit this in while the older one is out of the house as it's impossible otherwise). The rest of the day is pretty easy compared to the morning for me - if weather's good I walk to pick up DS1 with baby in the sling (fresh air and exercise is great, especially if you've had a bad night) and he'll have another nap (this is a great time to chat to DS1, hold hands and focus on only him). Sometimes baby's still asleep when we get back so I take that opportunity to have my lunch and do any other chores til he wakes. Afternoon is a juggle of keeping DS1 entertained, tending to the baby's needs and trying not to tire myself out too much. My evenings are long (DH doesn't get home til 9/10pm) so I have to pace myself. All meals are batch cooked and frozen so I never have to do any cooking when I'm home on my own. Baby has a couple of catnaps in the evening (in bouncy chair or on our bed) and I get DS1 ready for bed during one of those. Once he's down it's easy again til I go to bed!
To make your BFing sessions a bit calmer in terms of your DS, could you use the time to do something quiet with him that he enjoys? A lot of people recommend reading a book (held by the older child) while feeding, or if you have a hand free you could do drawing/colouring with him or play with cars etc? If he associated the feed time with an opportunity for him to have some time with you as well he may be less disruptive.
I know how daunting it is, but honestly you'll find after a week or two that you've got into your own routine without even realising and things will get easier the older your DD gets