It probably sounds kind of counterintuitive and like it might be disheartening, but I’m mostly self-taught, and I’ve always thought that doing tongue twisters helped me a lot. I don’t do them with any degree of speed, but I’ll go over a few tongue twisters several times a day, ones including my problem sounds and non-problem sounds. I like to vary the length, too, so for instance, I’ll do the “How much wood would a woodchuck...” one, which is long and not really full of my problem sounds, but I’ll also say “literally literary” bc subtle changes in middle sounds (PARTICULARLY r sounds in the middle of words) is a big issue for my speech, and it’s pretty short. I aim not to say each twister just once clearly, (I slur my words a lot as well as stuttering) so that I don’t get overwhelmed by trying to do the “three times fast” thing, and it makes me feel a lot more accomplished to do one of these super hard sentences/phrases than normal ones, and because they’re so hard, I don’t get quite so discouraged when I mess up on them. It helps me a quite a lot with confidence building! A lot of my self-improvement came from horrible boring exercises that made me feel stupid as a kid, too. I watched videos online, some of which were designed for people with speech impediments, and some of which were meant to teach foreign language speakers how to form unfamiliar sounds that occur in English. The premise of these videos was in teaching mouth movements/positions that should be happening for each sound and each sound transition. Where your tongue should start out and end, the different shapes your lips should form, etc. These videos mostly showed things that most natural speakers would never think about, I imagine, and that made me feel quite stupid, practicing mouth movements in front of a mirror, but I think it actually helped me form a much stronger base from which to improve, kind of like learning the alphabet before you learn to read. It might not be of quite as much benefit for you, since I think it mostly improved my slurring, which is caused by my dyspraxia, but I think it’s worth a try! I hope my response has helped you, at least a little!