She worked for ten years or she had been working for ten years. - both these are correct but mean slightly different things. She worked for 10 years = she worked for 10 years and now she doesn't work any more. She had been working for 10 years = she worked for 10 years then something happened eg. She had been working for 10 years before she had a baby. She had been working for 10 years as a waitress before she got a job as a chef. "Had been" is used to talk about something in the past that happened BEFORE something else in the past.

The correct versions are: 1. She had a great experience. (This means she had a great time. Please note that "experience" when used as a countable noun never takes the verb "get".) 2. The writer said she had never regretted working as an engineer. (Please note that "tell" needs an object: He told me/him/her/them that...)