Breastfeeding is a worthy pursuit but I really think that if it’s affecting your emotional or mental health, it is also worth it to quit. You need to be able to take care of yourself to be able to take care of your kids too!
I hope you find something that works. I am nearly four months postpartum with our first (no cycles yet) and we have been using Marquette with the addition of the Tempdrop device, which is worn on the arm and monitors temperature all night to give you the lowest temp in the morning. It learns when you are awake so that even if you are a restless sleeper or nursing at night, you can still temp accurately. We first learned STM (symptothermal) before marriage, but I didn’t really understand the postpartum rules well enough to feel comfortable using it.
I hope you find something that works! I can’t imagine six kids in seven years. May God bless you all abundantly!