Yep, okay that's a fair point. This is all in theory though, so say it was a seriously powerful and strong-willed Jedi (Yoda, or even Legends Luke perhaps? Or someone even better?). Could they resist?
This has been covered extensively several times. It's possible but extremely unlikely, carries incredible risk, and is not something that could feasibly happen more than a handful of times. It would take a very unquie character to pull off
I actually had a sith that was secretly a Jedi. She left the Jedi Order because she wanted to strike at the heart of the sith, but over time the temptation of the dark side and her own will torn her mind apart leaving her hollow on the inside. She fell to darkness even as her body physically rejected it, if she hadn't been killed I would have eventually had her go insane.
She kept telling herself that sacrificing her own soul would be worth it, if only to see the sith destroyed. Over and over she whispered such lies to herself, as if to heal her wounded heart or absolve her soul of her crimes. When death came for her, it was at the hands of her fellow Jedi, worse still was how she saw in them the same monster she had allowed herself to become. Poetry can be cruel.
In Legends Luke basically turns to the Dark Side and comes back in several different or continuous iterations but since the circumstances are so conflicting you can't really consider alternating between the two sides of the force too much.
Balance really isn't about finding the happy medium between the two, that area does exist but it is not balance, it is chaos. For a force user to maintain who they are without being torn apart they will find their own balance on one side or the other.