right, my DC2 is 6 weeks old and DC1 is 3 years old so I hear you - this is far far harder than last time! If your DS is anything like my DC2, then the heat is making him feed all the time.
cut yourself some slack, introduce some formula if you need to (it's not all or nothing, you could do formula when you are out and about).
Take your DD to places where you can sit while she plays, so parks that have contained play areas, soft play, so you can sit and feed, waving now and then while she plays.
Do you have a paddling pool? (there's one for £4 in Tescos!) Water, toys in the garden, let her get messy while you feed.
Will your DD be at preschool in September? That's only a couple of months away, it's only a couple of months when you will have two all day. Hold on to that!!!
If there's anyone who can help out a bit, even just taking DC1 out for a while to give you a break, take them up on it.
Remember, you don't need to parent DC2 the way you did with DC1, so if you don't BF, introduce a routine, leave DC2 a little longer when he cries, that's ok.