OMG, you are singing to the choir! I feel your pain, I think we have all had these times. Luckily for me they are short lived for a day or 2, not multiple weeks, yikes!
Things that I do that really helps me. My day is super structured. This helps as the kids know what to expect at all times and I have less battles on my hands as they know what they need to do. I have a lot of free play. I have struggled with this one as I feel I should be doing more but the kids are happy. The trick is that it is freedom within a structured day. This allows me to do the things I used to do at night which made my day longer e.g. chop fruit, prep lunch etc. We spend a lot of time outdoors, the kids like it and if not, they get used to it. I find the fresh air is a savior for my mental health. I go to bed early now (this I struggle with but I at least go upstairs to read). I started feeding the dcks the same things a couple times a week. They don't care and the parents don't ever ask/care and this has taken a lot of the pressure off of me. I started a sewing class one evening a week (sometimes it is very hard to go to it as I have "more important things to do" but I make myself go. Lastly, I let my husband take my girls to soccer every Sat morning and do what I like (usually cleaning) so I can relax the rest of the weekend. This was hard for me because I felt guilty not going but I got over it!