I have just started seeing an EMDR therapist and we have only just begun the preparation phase. I can tell this is quite different to any therapy I have had before, and I am blown away by how it has gone so far. In only the second session I shared quite a bit of stuff with the therapist and she was so skilled at containment strategies and calming techniques that it felt very safe. I have had more than a decade of therapy and am used to leaving the therapy room in a highly triggered state, but I left the second session completely trigger free, and have been able to use the techniques she has taught me thus far to de-trigger myself multiple times since then.
I am highly dissociative so we will do a lot of groundwork and preparation before getting into EMDR 'for real', but I trust that will take it slowly and keep it safe at every step of the way.
Up until three weeks ago I had never ever considered EMDR therapy. I've read a lot since then, and after only two sessions I can tell this is going to be a very effective took for me. My tip for lessening the fear going into it would be this: trust that they won't even start EMDR until it feels 'safe' to do it. You will do preparation work (learning and practicing grounding techniques etc) and not even start EMDR until that is done. SO you can start the process and preparation (which takes place over several sessions) and call a halt to the whole thing at any time.
For me, I see the preparation phase alone is so very helpful to me that I can't wait to do more.