Relationship between ED and loss of intensity of orgasm?

I've noticed a rapid decline in my sexual functioning. I am 42 years old and take medication to regulate my blood pressure, though it still runs high (140/95ish).

I'm finding that Viagra is necessary for me to obtain a solid and enduring erection. Also, when I orgasm, it doesn't feel as good as it used to, though my ability to ejaculate remains strong. Also, after ejaculating once and achieving another Viagra-induced erection shortly after, it takes a very long time to orgasm a second time if it even happens at all. A third time is out of the question.

I also used to masturbate 3-4 times a week and now it's maybe once every 7-10 days, though it doesn't really feel very good anymore.

Surely I can't be displaying these symptoms at just over 40 years old, can I? Could it be low testosterone or something treatable? The thought that the pleasure of sex will be substantially reduced for the rest of my life is quite depressing.