I've had a couple with my T and one big one with my marriage counselor. With my T, the first time, I waited until the next session to talk about it. The other time (a more minor one), I e-mailed her immediately after the session, she wrote right back, and it was resolved very quickly.

With my MC, it was a several-week process (wrote about it at length on here when it happened). When it first happened, I left a sobbing voicemail, then later that night sent him a lengthy e-mail outlining why I was upset (like I actually included bullet points). Then we discussed it next session, and I thought it felt resolved. A couple days later, I realized I was still upset, so we discussed it more the next session. Again, after a few days, felt upset again. So the next session, I laid out for him more clearly why I was upset, and he finally seemed to admit that he'd hurt me and that he'd made a mistake. I think that's what I needed to hear, so after that, our relationship basically returned to its usual close, secure, safe place.