My best advice is just to put yourself in seemingly awful situations and trying to focus on your trouble words or letters. I used to not be able to start sentences with H's or S's but through repetition and continuous substituting of words, I'm a lot better. I still stutter from time to time, I've gotten better at making them seem like natural pauses instead of trying to force my way through the words. The best way to build confidence is to just believe that you can speak fluently. And since you said your fluency has been better at times, just remember that period in your life. There was a time when I had to read plays aloud in school; some weeks I was the best reader in the class, and other weeks repeatedly fumbled on words like "and" and "but". Its all about flow and inertia, IMO. good luck man