Abby, what you are describing sounds like dissociation. Even though you can be with people you are comfortable with when it happens, dissociation is triggered by something that feels unsafe to you at some level, and your mind copes with it by going away. It could be related to a memory that gets triggered, and can be buried pretty deeply. My T has told me when I dissociate that I need to figure out what it was that scared me. This is work that really does need professional support / therapy to resolve. When you are able to understand the triggers and recognize that you are not in danger, you will have the control to stay present. SI often happens with dissociation, as a way to control it - both to numb out and to come back when numbing out is too uncomfortable.