I honestly feel like I'm totally failing at being a mum.
You're not failing, it sounds you are trying your hardest without any support 
My nearly 2 year old is so behind his peers to the point he can't talk, can't mimic animals and won't even attempt to 'play' with me to learn..
I feel like I just hit a brick wall every day when I try.
ICan have a really good progress checker for speech and language at 24 months. If you do it, it will tell you if you need to seek some further help.
It's also worth getting his hearing checked as things like Glue Ear can slow progress.
I'd also do the MChat as it will give you his risk for ASD.
I feel like I just hit a brick wall every day when I try.
My health visitor is pathetic.. hasn't taken interest in anything I have phoned about for help..
I feel like I'm slipping down a rabbit hole of depression and failing my 2yo completely.
It sounds as though you need some support fir how you're feeling. I'd book a double appointment with the GP. Ask the GP to refer him fir a hearing test and tell them how you are feeling. Talking therapies, medication or a combination of the two should help you get back on track.
I just feel like putting him in nursery every day so I don't even need to try now.
Don't feel like putting him in Nursery, especially if you can afford it. Would it help to put him in for two or three days a week and then plan some things for the other two or three days a week. Not necessarily things that will help him learn, just things that help you through the day like seeing family or meeting friends for coffee/walk around the park.
please don't be a twat.. I'm aware it's a terrible way to feel
And you didn't ask to feel like this, don't feel guilty that you do, just seek the help you need 