Erectile Dysfunction. But she is afraid.

Hi. When myself and my partner is planning to have sex and when we do foreplay, I am sexually and fully aroused. When I start also it is fine, well and good. But while having intercourse, sometimes in between my erection goes down. In my partner's vagina, mypenis is small and after some time it is aroused again. She was feeling pain and bit afraid. So I do allow her for insertion of my penis into her vagina. She sometimes feel I am not interested in her. I was bit deviated in between only once. So how can I recover from this and what are the tips. We are married for 3 months now. Sometimes she feels pain, so I do it slow. But sometimes she is not convinced. I tried to convince her that pain is normal. But she is afraid. Thanks. Please advise what to be done.