My ex was super into stand up comedy for awhile, he even attempted it once (oh god, the cringe), and he once told me over dinner that he'd listened to Bill Burr's podcast, that Burr recommended telling people "I'm just not having fun anymore" when you dump them, that it works at chilling them out, that they'll just accept it. I was like "Oh, huh, cool."

1 year later, he's dumping me for a co-worker after acting as though everything was completely normal, even discussing plans to marry the previous day. I was pissed, asking what the fuck had suddenly changed in 24 hours and I see an idea pop into his head. He looks excited for a second before he plasters a fake sad look on his dumb, twat face and says "I'm just not having fun anymore." Yeah, fuck off, dude, I'm not an idiot.

Just be honest, just tell them why you want out, explain how you reached that decision and why it makes the most sense to you to part ways, don't regurgitate a line that you hope will make them stop crying long enough for you to book it without feeling guilty, if you've spent any significant amount of time together, you do owe them some closure by giving them a legit reason. It's not just good for them, it benefits you because it helps them through the breakup process better.