I work as a qualified dental technician. I started about a year ago at my current lab and a few months after, a trainee joined. I work part time (30 hours a week) and she works full time.
She is hard-working and does have talent for her job. However, we both work in a relatively junior position within the lab (the plaster room). I have 4 years previous experience (in a similar role in a hospital) and already have full qualifications.
Outside of work we get on very well, but whenever I am given work in the main lab (to learn new skills), her mood quickly changes. She will stop talking to me and often complains about this to our manager. A few months ago, she lost her temper publicly with me in the office: she shouted at me that she was doing all the work in the plaster-room and that it wasn't fair (although the argument started over a petty complaint directed at me). Our managers sat us down later that afternoon to resolve the fight (and took my side), but this didn't resolve things.
My managers want me to learn new skills (and work in the main lab) and I want this too, but every time it makes me nervous because I know she will be cross about it. She is a naturally pushy person and now pushes our bosses to give her new work that involves learning new skills (which is fine), but at the same time she still fumes (openly to me) when I take on work. Today, she was being shown a new skill by a colleague and I asked to watch. I had a go (making a certain type of dental appliance) and she did not. When the colleague asked how the work had gone, she butted in and scorned my work in front of me to the colleague (he commented the work was quite good).
Moving forward, I want to resolve the situation. Soon I will be starting full-time in a slightly different role, with the plan of advancing my skills and moving up the lab hierarchy. Because she is so competitive and pushy, I am worried that she will basically elbow me aside - that I will end up doing the grunt-work that i already do and she will be the one to advance her career. In her mind, I think she sees me as a junior. She doesn't seem to appreciate that I am actually a lot more experienced (and qualified than her), even if I a still relatively junior within our company.
My managers are still not fully aware of the situation and I don't know how to raise the issue without seeming petty (which concerns me, because she does raise any petty complaint to them). I am not a naturally forceful personality like her.
What course of action can i take?