@Cinno raising 4 kids alone for a decade and counting, with no help does sound incredibly hard and I don't blame you for being worn down by it. DH's mum had to raise 3 kids on her own from when they were little BUT she had her mum on hand to help with childcare, and their dad paid maintenance and had them on alternate weekends. You don't have any of that so i can't see where you'd find time for yourself really.
The only thing I can say is that this isn't permanent. It does feel like a long time and that your best years are past but if you live till 80, 10-20 years of hardship doesn't define your life or future. And while your younger years may be past, they were not your best. Being older doesn't mean the quality of your years is worse, and actually it can be more liberating and easier getting all your parenting done when younger and reclaiming life once older.
The best way to deal is to stop thinking about the life you wanted. If you didn't know any other way of life, you wouldn't know what you have now is terrible. It's the comparison and past view that makes you focus on the shortcomings now. Accept your life now for what it is, focus on it being temporary (the way we all knew being the horrid teenage years were temporary), and find little joys each day to make it bearable. That won't be easy if you're not naturally a glass half full kinda person, but you can think of little wins you can incorporate every day that make the time go by sooner. Maybe gardening, or knitting/crochet, or learning a language, or playing a game online, or finding an online community for a hobby, joining a residents or community page on FB, starting a selling business of amazon or etsy, and getting more involved in a little project that your children can help with too.
There's lot of ways for cash and time strapped single parents to feel less isolated but it does take some imagination and more effort than feels fair. It is however the only option as there aren't any quick fixes until your DC are old enough to need you less, freeing up time to date.