Depends.
Is the issue extreme, genuinely? If so, then a working for communication and expression can be used. If you're a beginner, simply use a Solar mantra and the proper affirmation.
There are runes and mantras that are theoretically better, but I've found it's more effective to use a simple "all purpose" type of energy that you can feel better and connect with better. Each mantra/rune/whatever is an art of itself, and has certain specific characteristics.
Self-hypnosis can also help. Just make sure you're taking your sweet time designing the affirmations for it, I can't understate the surgical precision this practice needs, especially if you plan to do it for a while.
If the issue is moderate, just keep doing the routine stuff you should be doing anyway. A cleaner soul, protection against unwanted things, these help in everything, this included. Similarly so, the Throat Chakra, which you're supposed to be opening and empowering on a regular basis along other Chakras, will help with this issue over time.
Additionally, while it may be counterintuitive, don't worry about this problem so much. When we realize we have a certain flaw, we can start to identify with it too much. If someone has anxiety, for example, and they get a diagnostic that states they have anxiety, it suddenly gets "worse" for a bit (or longer), because they notice more, confirmation bias kicks in, and so on.
Inner problems people have been having for a long time don't necessarily get worse as often as they just occupy more of one's conscious mind, and loop around forever.
Suddenly, it's not just a minor deviation from balance, it's "this thing I HAVE". It's objectified, turned into some sort of miasma over the person. Even a thoughtform.