One thing that has helped me for many many years is the following quote:
"Never judge your insides by anyone else's outsides."
&
"No matter your life, there are those who would kill for having what you have."
You're jealous of 2 parent families.
I lived the same life you describe AS a 2 parent family.
Never a break, never time off (when I was sick I had to strap my baby in the car seat & we lived in the bathroom for 3 days while I had food poisoning). Of course, there were always the times when my now-ex would "get involved" long enough to put me or my children in the hospital. Fortunately he never stayed interested more than once or twice a year.
I was sooooo jealous of single parents who had their support systems DOWN. Who never had to deal with drunken angry men barging into the house at 3am to scream and punch holes in the walls. And then the next morning to have to get dressed, and smile, and play "happy family" for company coming over.
People now look at me and say "Oh.... You're so LUCKY to have 50/50 custody!" 
No. No I'm not. I only stayed married for as long as I did to protect my kids. I managed to PROVE the abuse in court (paupering myself in the process) and the courts gave him 50% custody, anyway.
I get phonecalls in the middle of the night with my children crying and screaming and begging me to come get them... And I can't. If I do, I'm met by the police who will arrest me for kidnapping/custodial interference... And I'll lose custody... And my kids will lose the only sanctuary away from their dad that they have.
My 11yo hung himself at his Dad's house.
But as hard as my life is, I know parents who would kill for it. Those whose children have died, that they would give ANYTHING for 6mo a year. Those whose exes took their children and ran, and it's been years since they've seen them. Those whose children are permanently brain damaged, or crippled,those whose children have been poisoned against them.
So I take a deep breath, and hug my children, and instead of screaming and crying and shaking that O have to hand them over each week to be abused (although I've done plenty of that)... I look at it that some kids have to deal with abuse 24/7/365. But MY kids have sanctuary, dammit. No matter how bad it gets at their dad's house at the end of the week they get to come home, and be safe.
__________
Is life hard?
Yep.
Poverty, pain, grief, despair, failure, disappointment... These are all parts of life.
But, what I've learned in my own life! is that we can make hard things harder... Or we can work to make hard things better.
Don't fall into the trap of "It's not that bad" because other people have it worse, and don't fall into the trap of "judging your insides by other people's outsides"... And thinking that if you jus had what THEY have, everything would be good.
Recognize what's hard,
And do what you can to make things better.
If you need time off... Find someone to trade childcare with (or a church moms day out program, neighborhood kid to babysit, college kid who'll stay all night PURELY for home cooked food & a place to study in peace & quiet).
DONT not take that time off "until" or "because".
(Until you can have a 2 parent family, because there's no money.... Until there's a better job, until they're older, until, until, until....because of this, because of that, because of me, because of him, because because because.).
Until & Because are the ENEMY.
Until&Because are lies.
No matter how hard something is... There is always SOMETHING we can do about it. Whether it's a physical thing (like figuring out time off, or changing custody), or a mental thing... Changing perspective.
Don't wait until, and don't do nothing because.
Make it better.
