I agree with @Zoceta .
I think I don't quite get as severe reaction as what you describe, but it is tough. It was tougher at first. There are rarely times when I don't cry most of the session, and that's okay. As far as shutting down, that has happened after few sessions. Generally I've gotten used to the fact that I'm digging into the worst of my issues, so it can't be easy.
That meant that sometimes I progress a lot, even though it's hard.
That meant sometimes we touch on something I can deal with, and for weeks my life is havoc. But I keep going to therapy, and talking about it.
Eventually something gets better again. It's a process, and it's hard.
I also always have in mind that it might be tough time right after therapy, so I usually make sure not to have things to do that can't be rescheduled. Or if I am already having a tough week, I even make sure that there are few hours after therapy with no plans, just the plan of taking care of myself.
In the weeks when therapy is really triggering to me, I am still trying to adjust. There are good days and horrible days, so I just try to adjust and do what I can on the good ones and give myself a break on the bad ones. Not that it's easy in any way.
The thing I've seen is...there will always be triggers, there is no way to completely avoid them. The only way to improve is to face them. But at your own pace. There have been weeks in therapy when all I have energy for is to talk about my regular mundane problems and nothing even close to my past. Also, there was quite a lot time when I was dreading therapy and just couldn't go. So I didn't.
I keep saying now that I wish I'd gone a lot time ago, but truth is I am not sure that was a possibility for a long time. So you have to really think and go with your gut on that one. If you decide to start therapy, it might be the hardest process, or it might have to be your priority over other things for quite a while. I actually didn't go until I had gotten things better myself, then got badly triggered and things started going downhill, I tried to get them better again for perhaps a full year. And only when it was so bad I couldn't get myself to see people, or go in buses, or call on phone, or go to dance, or even do my job, when all I was doing was trying to get by and survive, only then it hit me that at that point, I have no choice. I need therapy, or soon I really won't be able to get myself out of bed at all.