What would be a more acceptable way to say in a job interview that you resigned from the job because you can't keep a keen attention on details?

So I’ve been told to resign from my current job or they will fire me because I’ve made a lot of mistakes these past few 3–4 months on my new job (I don’t know with you but I think my employer is nice enough to suggest the option to make it appear like I resigned so I won’t look “sucky” when I get a new job). I guess they hired the wrong person and I guess I’m in the wrong job. The job requires keen attention on details because it deals with bank records and Anti-Money Laundering Policies which is very critical. So I’m worried lately because I can’t think of a creative way to tell my next potential employer the reason why I resigned from my current job. Any suggestions? Or should I better of just keep mum about this?