It can be really challenging, both physically and emotionally, to manage an enlarged prostrate and a high PSA when it comes to sex. It is great you are taking Viagra which is helping to get an erection, but it sounds like it is not as satisfying as you would like.
Whilst enlarged prostate and erectile dysfunction are separate problems, they are connected. Some drugs used to treat an enlarged prostate can cause difficulty in maintaining an erection. Your urologist will be able to provide specific information about any drugs you may be taking, and always check with them first before changing any medication.
Erectile dysfunction is a common problem for men of all ages and can be a result of a medical issue yet it can also have an underlying psychological cause. Through my work I have found there are three main underlying roots to psychological erectile dysfunction, and performance anxiety is one.
Unfortunately, once we start experiencing a problem when having sex, it becomes the thing we think about the most in any sexual experience. Taking the pressure of having an erection and relaxing into the experience of pleasure rather than penetration is a great way to start this.
You could try exploring other pleasurable sensations including mutual masturbation with your partner or oral sex that will help you feel hard and maintain your erection. The addition of external stimuli whilst engaging in penetration can often increase your arousal and take your focus off your erection. Perhaps you and your partner can bring role-play into the bedroom or discover sexy talk that turns you on.