okay,firstly,buy a sling,a wrap one like this
is there anyhing pushing on the area of the breast you have mastitus in? a bra strap for example?
yoiu can try expressing to ease the pain in the breast.
4 weeks is very young,and tbh you need to give yourself more time to get used to this,it's such a huge thing (2 kids) i have a 2.5yo and a 4mo.i feel like im getting back into the swing of things now(it began to get better after 2 months) but i have friends who are ff and are still a bit of a mess!
try not to feel to guilty about your dc1,is there anybody who can come to yours,and hold the baby whilst you play with your older dc,have a shower etc?
i found that asking for this rather than asking people to take my dd out made for a much happier dd,and ds didnt really care who was holding him,he just wanted to be held
as for your mother,tell her child care has moved on from when she was doing it,leaving babys to cry releses stress hormones and can make for a very stressed adult.
you are NOT making a rod for your own back,or any of that nonsense. 4 weeks ago you ds was cuddled up inside of you,he still needs alot of reasurance from you,and hoilding him is the best thing you can do for him.
im sure more posters will be far more eloquent and perhaps have more advice...i just couldnt let this go unanswered.
THINGS WILL GET BETTER!!
(you can pm me anytime for a rambling,badly spelled answer to anything!)