I used to dissociate constantly, and not only in therapy. Part of my PTSD symptoms. It is almost never an issue anymore, and even when it is, it is not as deep or problematic as it once was. In my case, I learned to recognize it when it was happening (just prior I guess) and how to stay present. Dissociation is a choice of sorts, but not consciously obviously. I had to learn to be more conscious of my use of dissociation as a means of protection and avoidance (which is what it served for me since I was about 5 years old). Took working through the issues that set off that reaction in me and practice, practice, practice over time but it certainly is something that can become more manageable.