There could be many possible reasons for this situation. It may result from a lack of experience; or it may be an instinctive physical reaction caused by long-standing behavioral habits. It could also be related to certain chronic conditions, such as high blood pressure—medications for managing chronic illnesses like antihypertensive drugs may lead to such issues.
In short, the causes are complex and diverse, and most likely a combination of multiple factors. I suggest you stop taking any medications targeted at this problem for the time being, and visit a hospital to seek professional medical advice on your medication regimen. In addition, you may try to adapt gradually, preferably with a familiar and steady partner, which will help you relax. You can also communicate with them in advance so they can cooperate with you.
