The inability to enúnciate specific vowels in certain sentences or situations is exactly what I deal with as well. I am 18, and have had a reoccurring problem with selective mutism and stuttering to get sentences out since I was about 7-8 years old. It got better when I was 14-16 and was able to speak to people more comfortably and make presentations in class and such but these past couple of years that I’ve just kind of been alone due to taking mainly online college courses, I am no longer comfortable speaking and often struggle to even respond to someone suddenly asking me a question, have group conversations with my friends, and even order food sometimes. And it’s because I’ve been so isolated. If you don’t want to go the therapy route which I’m sure would help, then as hard as it may seem, please try and get yourself out there. Start trying to talk to strangers, compliment a stranger, attempt to start a conversation with them. If you freeze or cant get a word out, the worst thing they can do is look at you weird but after that you never have to see them ever again. The best type of therapy is learning to get out of your comfort zone to get out of your current situation, hope this helped. I’m still trying to figure it out for myself right now but you’re young and you have time so go for it buddy.