Have you gone to see your doctor?
I think getting a good standard physical and blood work, and include those I have listed above and any other your Dr. would recommend. I think that would be a good place to start.
There are more and more men that are young like yourself who are getting Low testosterone. So don't let the Dr. talk you out of getting your testosterone checked. If nothing else it would be great to have your level checked when young to get a baseline if nothing else. I wish I had tested my Testosterone when I was in my 30's.
I would think that a mid-30's man would have a total Testosterone of about 600 or so. But every person is different and some men feel fine with total T in the upper 300's to 400. Others need to be much higher in the range.
When you say, starting to have issues in your marriage, what do you mean? How is the marriage otherwise, stress, job stress, depression?
Low Testosterone and low thyroid can cause fatigue and depression and both can kill sex drive.
your symptoms are pretty general so it really could be anything.
Get your testosterone checkes!
Need at least the following:
May want to get thyroid checked. To include
What other symptoms do you have? Fatigue, weight gain? Brain fog?