She quit her job on her soap opera and moved to California for Don because he wanted to, then he decided not to move and her career went nowhere in California. That's why her mother was telling her that she never should have left her soap.
Meh, yeah, that's what others have said--I disagree because Don's exact line from that episode was "I don't want you to give up your career, I don't want you to think that." To which, Meagan responded. "There have been all these opportunities in Hollywood, but I never even considered it because I thought it would make you go crazy."---The episode before, also, they made a point of showing how Megan was fucking up on the soap (Not listening to direction ect) So, really Don didn't tell her to leave her show, Meagan wanted to, she had been considering Hollywood for a while--just didn't pull the trigger because she was afraid to make Don mad. (You could make the argument she felt compelled due to wanting to stay close to Don...But that didn't keep her in New York once she felt like she wasn't in the wrong for wanting to be in LA.)
But beyond that. She got the ability to pursue acting from Don. She got her "big break" from Don's connections. In no way was Don ever a detriment to Meagan, at all--not even a little. Even if someone disagrees with the fact that Megan wanted to go to California and thinks Megan left her job "because of Don"--then they'd have to admit she only had that job because of Don anyway.
No part of the relationship, from a professional perspective (In acting), was bad for her in any reasonable examination of her status before and after. The only detriment to her being with Don was the emotional fall out of a break up. But on a professional level (And an economic one, frankly)? She wound up far in excess of her original position (Even without the million). Also, on a personal level, what did she waste on Don, 2 years? And in that two years she was able to take acting classes and build a resume through Don's connections (And live, without working, in Cali reading for work as she pleased)--so it's not like she "lost" those 2 years. I mean this isn't Betty here, this woman didn't give "the best years of her life" to a failed marriage that she spent cooped up in a house while her professional life essentially died. She didn't put all of her "professional eggs" into the marriage basket with Don, and then drop it.
That whole episode was just silly, it actually felt out of character. It took the worst of Megan's qualities I discussed earlier in the thread and amped them up to ten and made her a caricature of herself.