Speech therapy. You've already seen that it can work. Once you achieve fluency again, you need to do maintenance. In my case, at or about your age, I didn't put in the work to achieve fluency, and once I stopped speech therapy, I backslid. When I returned in my 20's, I put in a greater effort which resulted in greater fluency. I would do the work. I would practice in the car on the way to and from work every day. I would use my skills in daily conversation. I never attended any sort of program such as you described. I worked with a speech pathologist once a week (maybe twice a week in the beginning.) Same program in my teens as in my 20s. I experienced modest improvement in my teens over the course of a year or so. In my 20s I achieved rapid results in less than six months. I attribute the difference to me putting in a much greater effort in my 20s. Maybe maintenance for you is a weekly or monthly tune up with your speech therapist? While I prefer in person, telehealth appointments are much more widely available today than in 2018. I have a family member that attends twice a month meetings with a therapist via telehealth. The great thing is that you know you can achieve fluency.