Cuckold fantasy turned into obsession! Help me !

Good day everyone,first of all I want to say I am ashamed to post over here,but seems right now its my only solution to get rid of my problem which turned into nightmare.
It does not matter where I am from ,just wanna say that I am a handsome guy,22 years old and like any other guy I started masturbating from around age 12-13,at 18 I met this girl who was very kinky and I liked it ,she was into BDSM things and everything and we had alot of fun,no problems back then. When I split with her(Around 19 years old)I masturbated alot thinking at her,even 3 times per day,but then my nightmare started,the cuckold thing seemed for me very interesting,hot and naughty( of course,me being in the submissive spot) wanting to watch her with other guys with bigger penises. Whenever I jerked off to such kind of porn my erection was ROCK HARD,meanwhile I started dating other girls and I started to notice that I have low erections with them,which was very embarassing for me,they were very kind to me and they all said it can happen to anyone,of course,I told no girls I was with ( relationship or one night stands ) about my fantasy. I had around 8 girls until now and with each of them I had erection problems,I start by being very hard,but my erection goes to about half after 5-10 minutes , meaning I cannot have sexual penetration. I am almost 23 years old and 5 years at least into this nightmare I made myself in the head. I cannot watch normal porn because I do not get hard without thinking that the women in the video are one of my crushes or ex's. I am even afraid to go out to bars and clubs,because I know that girls hit and flirt on me and most of the times they invite me to their place but I always find stupid excuses not to go,because I know I will embarass myself by not getting fully hard.I feel that is gonna be extremely hard for me because ive tied my fantasy with real life . Its so embedded in my mind now that i wont be good in bed anymore so the thought of even attempting it with anyone already makes me scared and panicking that it wont work .I cant say my penis is small,when I last measured it ,it was around 16 cm ,so I guess thats somewhere around 6 inches.I never want this fantasy turned into obsession to become reality,everytime I jerk off and cum to cuckold porn,I blame and curse myself for what Ive become.I tried to stop watching porn,but after 7 days I feel this urge to watch and lower the pressure. For instance,I knew with a girl that we were about to have sex and I even took Viagra pill,which had a very great and good effect,being hard for at least 1 hour and had awesome sex,but I do not want to take this pills everytime I have sex.Thats all I can think to say right now. Sorry for my bad english . Please help me ,I want to be cured,I want to have normal sex. Thank you people,I will wait for replies and responds.