I know exactly how you feel, you are not alone. I am 23, I also have a very bad blocking stammer that is 100% psychological, it makes life very hard. The most important thing I can tell you, is don't back down from opportunities because of your stammer. I hid away in my bedroom throughout all of University, I never went out to parties or anything. Don't be like me, sometimes forcing yourself outside of your comfort zone can help a lot. I know its hard. Try to not overthink everything, I used to do that. I was worried what people thought of me, actually worrying makes it worse. Now I am more relaxed, it helps a lot