Talk to Sexologist on Erectile Dysfunction

I am experiencing a specific pattern of erectile dysfunction. I can achieve and maintain a completely normal and firm erection for up to 30 minutes while lying down in a supine position. However, the moment I stand up or sit down, the erection completely subsides and vanishes almost immediately. ​I would like to know if this sudden loss of erection upon changing positions (from lying down to standing) could be caused by psychological anxiety, or if it points toward a physical/vascular issue like a venous leak or pelvic floor weakness. Please guide me on the next steps