My penis wont harden a bit if i touch it or fantasize about girls. :(

It started around the end of last year, It was so sudden like my penis wont respond to my thoughts or touch. Like its brain dead, but when im with my girlfriend it hardens automatically. The problem is when i start the penetration its not 100% hard then after like 3 to 10 mins it eventually gets limp even if i havent ejacuated yet.. Its so embarassing when that happens. Im 18 now and i started watching porn since 12 years old than had my first girl at 15.. Idk why suddenly my penis is acting diffrent. Any advice guys??