I think any age mums who work minimum wage jobs or similar do actually have it really hard if they want to work. Not everyone has the ability/qualifications to do a job that means you come out with a profit that pays for your actual outgoings plus the costs of your daily life. Nursery costs are really expensive even if you do have a good job.
I see where OP is coming from. Some people are fortunate to have family who can help out. Some are not so fortunate. It isn't OP's fault that she wasn't lucky enough to have family to help out.
If you work a minimum wage job (and don't forget that minimum wage is less if you are under 18) then it might not even cover the costs of nursery. Minimum wage for 16-17 year olds is currently £8 per hour. For 18-20 year olds it's less than £11. Even for those over 20 it's only just under £13 an hour.
Nurseries often work out around £10 per hour or more in more expensive areas. Cheap areas aren't that cheap where nurseries are concerned, even if there is a place! A younger mum working a minimum wage job and topping up anyway with UC is not actually going to be seeing much of a profit from working. It's very easy for those of us who have good qualifications to say 'well, get a better job'. It isn't always as easy as that.
And those minimum wage jobs are keeping the country running. Shops, caring roles, factories etc. People in jobs like this may well need to claim benefits to give their family a basic minimum quality of life. It isn't their fault.
The entire country is being subsidised by the govt, not because people don't want to work but because the jobs available don't pay a decent wage and those doing them will need to claim benefits anyway. Why are we subsidising companies that make huge profits? Why aren't they paying a fair living wage?
I haven't worked a minimum wage job since I was a student, so no skin in the game. But I understand that it isn't as easy for everyone as it was for me (and it wasn't that easy for me tbh).