I would practise mediating it calms you down helps manage anxiety and your speech, their this app called” waking up” its free all free and you do meditating for 5 mins one day then day2 same thing like progressing. This will make you feel much more calmer. Thats what its done for me anyways. Ive been stuttering and stammering since i was 11 im now 19 and its improved quite abit also. Another thing is when your on online classes and stuff dont be afraid or anxious that your going to stammer accept that you have a stammer be more open about. Like say to them oh i have a stammer that way if they ask oh why you taking so long to answer or something like that. I know what you mean the anxious and the heaviness in the chest i get that also. Also when you wanna say something but physically cant, try breathing and then on the exhale say what you wanna say. And practice like diaphragm exercises also. Also maybe start reading out loud to your family to practice speaking out loud to large groups. I know you already kinda said you do but just so that this improves your confidence, when your speaking in loud groups also make eye contact with people when your speaking with them one on one im guilty of not making eye contact with people one on one but im trying to improve with it this will also help you confidence levels aswell. Anyways Hope this helps you and whoever else see this comment have a good day everyone stay safe.