19 yr old can't get erect for sex

I am 19 years old and have had my girlfriend for 5 months today. Before her, I dated a girl for 2 years and she was my first and we had a lot of great sex but sometimes I couldnt get hard but it was rare. Since her and I broke up I've tried to have sex with 4 other girls and failed each time since I could not get hard. Now with my new girlfriend, she is getting frustrated at me that I cant get hard for sex. If she is giving me oral sex it's fine and I ejaculate everytime, but the second I go for the condom, my erection just goes away. With my ex we didnt use condoms since she was on birth control and we both were virgins. I really need help, before she leaves me or just for the future of my sex life. Im a healthy 19 year old male who eats very well and runs all the time. I know most people say stress but ive been trying for months now to releive stress and have a different mindset when trying to perform.