The only real difference in a couple is that it is possible to get a bit of headspace. But in our case, we both work, although I'm part time, DD goes to nursery.
On a morning because of DHs commute he has to leave before nursery opens so all nursery runs are on me before I then have to walk the dog and get myself to work. He also gets back later, not through choice I might add, so although he aims to do the pick up I often need to because of traffic. Which means I finish work, walk the dog again (v high energy breed, which we had before child), prep anything I can for dinner then go get DD. Sometimes if I've had a later finish I have to finish work and go straight for her.
Then, much like with yourself, it's dinner, bath, bed, housework, MAYBE a bit of downtime if all goes to plan, bed for us then start all over again.
So yes, some elements are easier when there's two of you. But my DD is fairly happy, easy going and easy to deal with and it's still full on all day. So it's only because we can each take a job when we're both in the house that makes it physically easier than for a single parent.
I imagine your mental load is higher as you're taking on all the worries as well as all the jobs.
However, people have different worries and problems. Couples are obviously going to have issues between themselves, at least sometimes, because no two adults can live together and not occasionally fight. You don't have that to deal with day to day. Additionally, their child may not be happy and easy going. They could be prone to illness. They could have additional needs.
Everyone is in their own situation. No one situation is perfect. And no one person handles things the same.
You have a very "hard done by" attitude. Yes you're in a hard situation, but so are many others. Try and see the positives rather than always expressing the negatives.