I get it. I get 2 days (days only, no overnights and I still have to do the drop off) a month - but most of the time I just spend that time catching up on things that I haven't had a chance to do the rest of the time with kids and work (because I work around the kids, so that fills up more time than just the day).
It's relentless, and the type of relentlessness just changes as they get older.
I used to have evenings once they were in bed, but now they're bigger, and I work earlier, so they stay up as late as me, and I feel like I have no time alone, to myself at all - especially since the youngest has decided that he likes to wind-down in my bed (I used to spend an hour before bed just pottering in my room, but now he's there, so it's not the same). Plus my eldest likes a hug before he goes to sleep, so I can't go to sleep earlier, as he'll just wake me up when he comes for the hug (he'd just turn around and go back to his room, but I'm a light sleeper) - and I don't want to discourage that, because it's not forever, and they both need the security.
I think you just need to be as kind to yourself as you can, when you can. I refuse to give myself a hard time about the occasional takeaway, or skipping doing the dishwasher for a couple of days, or having an occasional afternoon nap.