Being Promoted and then Demoted?

[Merthyr]

I work in a fast food restuarant as was recently promoted in March to a Trainer,We have regualar monthly meetings,I am meeting most of my targets complete alot of training logs,I do think theres worse people in the position than me,A few of the trainers are going in for manager which i dont think is right,Everytime im in work it feels like im doing a crap job ,My training manager has told us that she would be looking at who is doing their job properly in January to see if their right for the job anymore but she said she is not trying to scare us with demotion?,she says alot of things and doesnt take them out mind ,I always work on and come in on my days off and can work on most stations in the resturant.I am constantly making mistakes though ,ive been going to bed very late though and i dont know if this is anything to do with it?
ANy advice would be great thanks