How do you get over success?
There was a program hosted by William Shatner, the original James Tiberius Kirk of Star Trek fame, where he interviewed all of the Enterprise captains and they all had the same problem - what do you do after you have become known for something? There were only 79 episodes of the original Star Trek and Shatner was in the last 78 of them, but they defined his life forever after. He finally decided that he wouldn't fight it - when some fan said "Beam me up", instead of getting irritated as he had originally been, he would just say, "I would if I could" and he could finally live with it.
If you are in a creative business of any kind, you will have projects that stand out in your career and other projects where you were just marking time. Sarah has been one of very few actresses to become an iconic figure in a long-running popular series. She has made enough money to last the rest of her life if she doesn't spend like an idiot. I have had a long career in design engineering. The high points were spacecraft in 1973 - 1976, cockpit displays in 1988, design auditing in 2003 and a patent that issued in 2000. The rest of the time, I had work that paid the bills, but that was OK - you can't have high points without low points.
Sarah comes from a soap opera background and this carried on through Buffy. She could emote with the best of them. Anyone in the Whedonverse has aced an intelligence test - he only hires people who can read the script and bring something to the table. Sure, she will always be Buffy to everyone on this board, but time has passed, new projects keep coming and she will adapt and find new fans in another project.
David Boreanaz is probably known as FBI agent Seely Booth on "Bones" more than as Angel, Alison Hannigan is probably better known for, "One time, at band camp..." and is now Lily Aldrin on "How I Met Your Mother", a comedy that appears to be in stealth mode is still paying work, just like her shampoo commercials. It may take longer for Sarah to develop another persona, but it has been almost a decade - she should be able to do pretty much anything at this point.