Hi, I Am 24 Years Old. I Used To Masturbate

Hi,
I am 24 years old. I used to masturbate regularly in prone fashion for past 8-10 years. Now I have stopped prone since past 3 months. I have morning erections but still I found it difficult to keep it hard for 2 minutes when I tried with my girlfriend. My psychiatrist recommended me Tazzle (20mg) (Tadalafil). I have never tried any such thing. I am worried about its side effects and I was wondering about the proper dosage
Thanks in Advance