Normal sex with regular partner , not able to get erection with new girls

I need some advice from either a therapist or anyone else.

I can have sex most of the time with my regular partner. Most of the time I get hard and do sex in morning. I often wake up hard in the morning. I used viagra also and it works great.

However when it comes to any new girl I can not get erection. I tried everything and with beautiful girls. I tried viagra also.

Things work with regular partner but not with other. Any suggestions?