Doesn't "Sarah, je dois te dire ce qui m’arrive" work?
Yes, but when you're sharing an experience, or telling a story, raconter is the better verb, think of it like you're recounting a story to someone
Why isn't "ce qui me passe" not even considered in the valid answers?
"Sarah, il faut te dire ce qui m'arrive" not accepted.
Yes, actually your sentence doesn't really work. Here you can't use the impersonal form il faut. It sounds too much like you must. And you could not say You must tell you what happened to me.