I Have Been Experiencing Sexual Difficulty For A Few Years

I have been experiencing sexual difficulty for a few years now, and I as of recent I'm taken the time to try to find out why things are happening.. for the 2nd time. I basically have a slow responding sex drive; low libido that used to be stronger (or NORMAL if you ask me)

1st time I tried to do something about it, I went to a urologist a couple times and they done some tests on me and some blood work. my testosterone levels where within range (600) and they gave me a sonogram on my penis.. in which left me with a FULL BLOWN erection for 8 hrs afterward. This occurred in 2013 where the onset of everything was fairly recent. I Was given a sample of cialis, and it worked the few times I took it

the 2nd time I tried, a month ago, I went to an endocrinologist to see if I can get help for my symptoms. He did blood work and tested for my testosterone (637 in normal range) free testosterone (155.5, a little above normal range) and Vit D levels ( 26, lower than normal range). He also tested TSH, FSH, and prolactin which were also within normal range. He also went further and did a sonogram on my thyroid (because he thought he felt it when he palpated around my neck) and found a benign nodule. I was advised I had nothing to worry about as far as that goes.

Since the tests he done indicate everything seems normal, but i don't FEEL normal despite it. I still dont get morning erections anymore, and if I do they aren't as hard as they used to be. It takes half as long to reach orgasm during intercourse, and thats IF I can maintain my erection long enough to put a condom on and penetrate someone. I generally don't feel a sense of arousal and deep WANT to partake in intercourse as I used to, so my question is this:

What is the next step I should take in figuring out why my libido isn't what it used to be? is there more tests that can be done? or is there another specialist I can talk to that will help?