I'm not quite sure if I understood your situation. You've been demoted to dev position in the new company, or if you went back to the previous?
In the first case I think that your chances are very poor - I suggest to keep looking new company.
In the second one you can try to convince your superiors that you want to learn and that you'll be good at this position..
You should think about whether your company is truly a place where your abilities are adequate. You wrote that it is a big company, but there are just a few developers.
PM's work is to manage the project- not a team - and the job is completely different than it may appear. I know because I went through the path similar to yours and became PM from dev.
If you find that you feel good in this role and really want to do it, I recommend you talk to your superiors, and convince them to your idea.
Otherwise, to do some training on the side such as PRINCE2 or PMBoK and then take off to another company with documented skills.
Good luck - fingers crossed