My older DD is nursing again after a 5ish month break during pregnancy, and I'm feeling some of the same feelings as you. She's back at it with renewed excitement and it can be really trying.
Here are some things I've been trying to do. I'm trying to remember them all. I feel like I've had to put way more thought into this than I expected... and some moments I feel just like you. And others I feel so happy that they are both nursing.
- time wakings in the morning so I can nurse DD1 (S) in her bed alone (DD2 is in our bed at that time), 'filling her up' with solo mama-love. I find it starts the day off much much better if I can nurse her by herself in her own space and she wakes up slowly with lots of attention.
- I almost never nurse them together. It's way too stimulating for me and every little touch or movement by S makes it worse. I end up holding her hands still and snapping at her if we do it, which makes me feel resentful and she's not getting what she needs out of nursing which is attention and closeness. She's getting a lot better about taking turns.
- I went to the monthly meeting of our local La Leche League last month (even though I had to force a tantruming S into the carseat to get there) and I met 3 other moms who've tandem nursed. Just talking to them made me feel 100 times better and now I know there's somewhere I can go to unload a little, which feels really good and S had a fabulous time playing with the 2 other 'big kids'
- I'm reading Adventures in Tandem Nursing, and it talks about giving the older child undivided 10 minutes as soon as the newborn is asleep, before doing the laundry or making that one phone call etc. I'm not very good at this, but I'm trying and it does make a difference.
- talking a lot to S about her own babyhood and how she got to nurse all by herself for so long, we made an album on iphoto with pictures of her breastfeeding as a newborn and baby and of her starting to eat food, talking about how Z needs milk to grow so she can get big enough to play with.
- I try to give S as long a session in the morning as she wants, but the rest of them I have to limit them somewhat or she'd stay on forever and I can't take that on top of nursing a growth spurting infant, even if Z would let me sit with S all day. I sing the ABC song or Twinkle Twinkle, or say she can have 5 sucks (and I count them out on her fingers, giving her a little hand massage at the same time). S always wants to try both sides so it's often 5 sucks per side (and both hands massaged). By bedtime I'm seriously touched out so I'm really firm about these limits. And maybe because she learned to fall asleep without nursing during my pregnancy, she seems to be ok with it.
- I bought a new water bottle with a straw spout, so I don't have to undo anything to drink. I take it everywhere with me and try to always keep it filled.
- S loves almond milk (the vanilla one with sugar in it) so if I really don't feel up for nursing her that's what I offer (it's a big treat for her) and I try to make sure I cuddle her and read a book with her close by the same time.
- S just stopped napping consistently, but I agree with trying to lie down and nap-nurse the baby once during the day. It's so much easier to be patient with the older kid when you're not exhausted. I've been putting on a 30 min video here and there to get some time to do this. I find myself feeling really easily frustrated and snappish at her when I'm tired or hungry.
That's a long list and I'm sure there are more things I'm doing to try to balance this ever-evolving relationship. It's not easy but I do think it's worth giving it a go trying some boundaries/self-care for you before trying to wean cold-turkey. Your older DD has just had the hugest transition of her life and if you do end up weaning her, going a slower, gentler route is sure to be a lot easier on her.
Big hugs and hope you can find some real-life support to take more breaks and recharge yourself so you have the stores to give both your kids.