English being my second language, I still sometimes struggle with it if I speak my mother tongue exclusively for too long. I swear by this "trick" and it's helped many people I know with their new languages. Never analyzed it too hard in terms of how it connects your cognitions but it works like a charm: Read. Out. Loud.

Simply reading anything out loud improves my speed, accuracy, and tonality significantly and instantly. I'm talking first-language-for-a-week followed by frustrating stuttering to absolutely coherent smooth fluent speaking within 20 mins of reading. You can focus what you want to work on of course.

If it's speed then read very fast and skip mistakes in pronunciation just keep reading and spewing it out your mouth as fast as you can. If you're trying to control your speed as in introduce pregnant pauses, stop at meaningful parts of the sentence, talk faster during certain parts then slowing down in important parts, or whatever the style you want to introduce is, do it very consciously. What also helps if you're struggling is watching a video of someone with the voice style you're aiming for before reading. This instills the voice you want in your mind so you can be a better judge of your physical voice. Try someone polarizing (as in on the other end of your extreme) first. I usually use RSD Jeffy for his pregnant pauses and strong sentence endings, sometimes a fast-talker like Russell Brand if I notice my voice is getting too monotonous. Also practice on different languages, I practiced game a lot in English so my tonality is getting solid but my first language sounds like an excited child.

If it's pronunciation and accuracy, read very slow and enunciate as carefully as you can, don't eat up vowels, move your lips too much on consonants, make it so crisp and slow but very steady, then escalate the speed of reading gradually every week.

If it's your accent (which is the hardest but completely achievable through perseverance for a few months), try reading something then listening to the correct accent. For instance if you're into British accents, watch Top Gear with english subtitles. It's an exercise so don't go thinking it'll be that much fun. Every few seconds, you pause, read the subtitles in your best British accent, then play to hear how Jezza said it. Don't replay the sentence if you got it completely wrong or if you wanted to notice some sound he makes, just keep going and that sound will happen again and again. When you get good at this, play the video continuously with the subtitles on a negative 2 second delay (subtitles appear early, use MPlayerX or VLC). This way you can read the sentence as he speaks it and see if it matches your voice.

Reading the fuck out loud. You read shit alone all the time anyway, TRP for instance. It also helps if you're in isolation for a long time, just getting your brain into the communicating-outward gear. You'd think it's simple as fuck but like kegel exercises, you just have to remember to do it daily.