Okay - working mom of 3 (6, 4, 1) here.
1) Almost all of us are exhausted most of the time. You’re not weird, and I doubt anything is wrong with you.
2) 18 months is hard. It’ll get easier every year.
3) No one is doing “it all.” Make sure you’re cutting corners, it’s the only way to stay sane. I have given up on the following things: Christmas cards, thank you cards (I text), washing sheets more than every three or four weeks, wearing cute outfits, doing my hair in any way that takes more than 3 mins, showering daily (it’s every other day now unless I’m sweaty), shaving my legs in winter, anything in the category of “redecorating” and probably about 294729 other things that are just objectively less important than staying sane, doing a decent job at work, and raising my kids well. And these things don’t even bother me. It’s honestly freeing.
4) You need breaks from the grind. My husband and I aim to each take a chunk of time completely off from childcare responsibilities most weekends. The other parent is 100% in charge during that time. Today, I slept until 10am and then laid in bed and watched bed TV and played on my phone until noon while he was in charge of the kids. This evening, I took care of the kids, did bath time, dinner time and bedtime solo, and my husband went to a movie with friends. To repeat: this is not a once in a while thing. It’s nearly every weekend, and it’s CRITICAL for staying sane. We also try to each take one or two weekday evenings off per month to have dinner with friends or go to a happy hour or show or something. Additional advantage: while taking care of all the kids solo for long stretches can be challenging, it also makes all the other times, when your spouse is there, seem easier.
5) Can we dig in more on your evening routine? Kids in bed by 8, great. Cleaning, etc, sure. But then 20 mins later you’re asleep? Something here isn’t adding up. Are your cleaning/evening chores taking more than 45 mins or an hour? Each? Every night? Then see #3. You’re doing too much and need to cut way back or simplify or something. We might be able to give you tips if you can tell us some of the time sucks there. Otherwise - does that mean you’re falling asleep at 9pm? What time are you getting up??
Assuming you’re up around 7, you should be able to be asleep around 11, getting ready for bed around 10:30, which should mean you have from 8:30-10:30 to yourself just about every night. Provide more detail as to what’s going on here. My suspicion is you are spending a lot of time in the evenings on screens, which isn’t awful sometimes, but if it’s robbing you of all your evening free time, you’ve gotta restructure your habits/routines.