You've gotten some pretty good advice here, but I am going to take it another direction.
How much porn do you consume? How do you watch it? Today's social media, swipe right/left, instant access culture will wire your brain and lead to the very thing you're experiencing. The result of months of instant access is frustration, lack of stimulation, and general unhappiness with life. If this is what is happening/happened to you, I would suggest going on a porn fast for a month or more. Replace the times you are looking at it with reading well written erotica. Reading about sex engages the imagination which may rewire your brain back to a point where visual stimulation is once again a turn on.
BJ.