The reason you wait until the last minute to make changes is probably the same as for lots of people. You think whatever it is will get better on its own. It’s painful to make changes so you put it off. Your rut is familiar even if it hurts you. You are pessimistic that you can do it. Even though you’ve been around this track a few times, you somehow manage to persuade yourself that this time it will be different. Right? Right.
But this time you really do want to do things differently. Share your worry about backsliding with your therapist and ask for help. Ask your therapist to collaborate with you in making a “maintenance plan.” Agree that you won’t leave regular treatment until the changes you make are at least 6 months old. You don’t necessarily need to go every week (though maybe you do). But you do need to keep going long enough to make sure your new approach has become a new habit. I would then suggest quarterly checkup appointments for the next year. If you get to within a week or two of the appointment and things are going fine, you can always cancel. But having the appointment may help you stay on track.
I wish you well.