I also faced a problem similar to yours. When I’m not under pressure and use a crutch it works perfectly and I really enjoyed the way I spoke and was cherishing the fluent memories but under pressure I did not do well and I seldom stuttered and this broke my heart and that was my first and last relapse. First thing, after becoming fluent I was over confident and did not do my AST properly and the second thing was I COMPLETELY FORGOT TO USE A CRUTCH UNDER PRESSURE.
This is a common mistake every PWS makes, they simply forget to use the crutch under pressure. So what do you need to do? All you have to do is practice the crutches all time and every time you speak even if you don’t need one just like Coach Javier had already mentioned. This helps you to blend the crutches into your system. Once you have mastered the crutches you will use them in your speech automatically. It will become your speaking style. You can also observe the way non-stutterers speak, you can always relate a crutch to their speaking style. I do this often and it is fun and it also helps me in remembering the crutches.
If you are not sure of how to use a crutch then you can sign up for stuttering plan and watch the videos wherein Lee explains each and every crutch in more detail.
All the best,