Effect of SSRIs on brain. Take this med?

I have been suffering from social anxiety, generalized anxiety, emotional hypersensitivity from the age of 16. I'm 21 now, I went to psychiatrist 3 months ago, he prescribed me 25mg dose of paroxetine and 20mg dose of propranolol daily and nothing else. I saw a drastic change in my personality in the last 3 months. I'm no more depressed now, more outgoing, feels positive, rare mood swings, and a complete halt to my overthinking. I was just curious know if paroxetine permanently alter brain chemicals and function or I'll get back to real me after I stop taking this med. Also how long will I need to take this med? It's 3 months and I don't think I'm completely cured, I still have some social anxiety which make me stammer which is very embarrassing for me.