I feel overwhelmed with my two and I'm not a single parent so I take my hat off to you.
I think you need to create me time. Look into some kind of daycare. It can be at a creche or by a family day carer. Your council should have details about family day care or ask your MCHN. Our local council also runs occasional care from the local community center. It's a 3 hour drop off and it's just perfect for getting the groceries done and having a relaxing coffee and cake in a coffee shop (which is way luxurious for me). And it costs $10 per child so it's not going to break the bank.
Comments like what was said by your mum are just plain horrible. My children were very much wanted. That still doesn't make any difference to me needing some time to myself for 3 hours once a week!