You could try speaking to your line manager and explaining what you are finding difficult about the role. Don't just wait for he/she to tell you what to do but go into the meeting with solutions -eg i am struggling to do X, Y, Z, perhaps I could job shadow you/have a colleague as a mentor etc. When confronted with a piece of work try and look a little wider, think about the impact that the work might have on other issues, other people, what might be the outcomes etc and maybe start considering those issues alongside.
I've been in a similar situation and now have a post- it on my desk reminding me to think of the imact on related issues, horizon scan (think about what's coming up in the future). Don't be afraid to ask for help but don't be passive about it, take ideas of your own. Expecting you to do the work without help is a fault of your managers - not yours!
Finally, are there colleagues you could ask for help? Maybe someone who does a similar job and does it well. You could see how they cope with the work and try and learn from them.
Good luck, it's a depressing situation to be in but it can be turned around.