Thank you so much for sharing this. I don't know if I really have anything useful to offer here. But I noticed you had not yet received replies to this post. So I thought I would at least offer a suggestion. I'm not sure if I clearly understand what you're asking for here though. So if I miss the mark with my comment please forgive me.

I wonder if you could develop a list of attributes you would consider to be adult... things you might do or ways you might live your day-to-day life that you would view as being adult. And then, also, figure out how much time you're currently spending on average day-in & day-out as a "little" as well as how much time you're spending as an adult. Then develop a "schedule", so to speak, where you gradually increase the amount of time you spend doing things from your "adult list" while decreasing the amount of time you spend being little. It would be sort-of like titrating off of one medication while gradually increasing another. Your therapist could, no doubt, help you with this.

I presume this needs to be a gradual process. And you could be flexible with your schedule. If you reach a point where you feel you need to put things on hold for a period of time you could do that. But having a "schedule" such as this would give you a way of measuring how far you've come. You could even plan some rewards into the schedule periodically for having achieved certain percentages of progress. This would be kind-of a behavior modification approach to what you are trying to achieve.