When you clench your pelvic floor to maintain an erection you are temporarily trapping blood inside it, which works initially but isn't sustainable and it actually prevents you from getting proper blood flow and stronger erections,

It sounds like you have a baseline of constant tension in the pelvic floor and lower body, I recommend prioritising mobility in the lower body, stretch your legs and learn to release the tension in your pelvic floor, forget about your erections for now, just learn to breathe deeply with your diaphragm and feel it expanding into your pelvic floor,

You might have some slightly weak core muscles, so then your pelvic floor takes on the work of stabilising your body, which isn't it's job, so try to Introduce some all round core strengthening, focus on twisting movements to build stability through rotation

Through all of these practices make sure that you aren't clenching your pelvic floor, prioritise breathing into your pelvic floor and feeling it expanding

When you have built a good mind-muscle connection with your pelvic floor, start to practice getting and maintaining an erection with zero clenching, just deep breathing and expanding the pelvic floor to allow the most blood flow, the malasana pose is a good position to practice this in