The first thing is to see a Speech and Language Therapist for an assessment to see if there is a physical problem with your speech. From the way you describe things then it is possible that you have a social phobia which is the underlying matter. But the key issue is to get an independent evaluation.
However if you have any other problems such as weakness in the hands, unsteadiness walking or loss of sensation any part of your body then you need to see a doctor as soon as possible for a full neurological examintion.
At your age, then the most likely explanation is stress and anxiety so some simple techniques for relaxation may be helpful, as well as practice vocal exercises.
I suspect the underlying cause is a 'multifactorial' which means many things could have contributed to your current situation. Certainly it has been shown that use of some drugs (such as cannabis) increases risk of some mental health issues.
However a social phobia can be sorted if you are prepared to put some time and effort into first seeing a a counsellor, to explore underlying issues and undertake cognitive behavioural therapy and also start medication such as citalopram which is very good for social phobia but takes some time to be effective...so stick with it. You will then feel more confident about entering social events and over time, things will improve.