God, if only it were that easy. I had this issue with a FWB, and we went through a few distinct phases...
The first was just reassuring her I didn't mind. Listening and responding is easy enough for me, but I'm a quiet person, so it's a great thing for me to have both someone that has a job/life that gives her a lot to talk about every day, and someone that has the social battery to talk about it. When the apologies persisted, I tried being more and more precise and long-winded in my explanation...to no avail.
Which did get exhausting, admittedly. So the third phase was scaling it back. "Like I always say, I like listening to you and don't mind at all". Which I guess was my way of politely saying "I'm running out of ways to say this."
The fourth "phase" was her putting it more...bluntly I guess. "Sorry, I know I'm annoying you so I'll see if I can shut up for just two minutes." Which was frustrating for me, because I've already done everything I could think to do to reassure her, but she doubled down and that made me feel like there was truly nothing I could do to get through to her.
That's all I had to say I guess. Thanks for coming to my ted talk.