Why can't I maintain an erection? I'm 46 good blood pressure and heart date. This is very disconcerting

Erectile dysfunction. ED can be caused by depression, anxiety, high blood pressure, high cholesterol, obesity, spinal cord injury, prostate cancer treatment, smoking, drinking, low testosterone level, and anatomical problems such as a hydrocele. There is treatment and you should be worked up.
Your doc. Or a natural doc can test your hormone levels and rule out circulatory issues. Also, a clinical psychologist can teach you skills to keep from focusing on your "performance" which interferes also. Peace and good health.