I don't think your problems in the bedroom are necessarily related to BP. I also wouldn't push this off on "mental" without having more information (like a history of depression, anxiety, or something).
If we were to focus strictly on your concern of weak erections (without diet, BP, etc.), it sounds like ED.
I am not familiar with removing starch from the diet. Does that include breads and other high carbs? You need carbs for energy and you say you don't have as much energy.
Less energy coupled with possible ED is indicative of a low Testosterone level but it really can't hurt to have a full T workup (not just Total T!).
I was having similar symptoms as you. Even though I had morning wood, it was infrequent and was brief. I could get it up when I wanted but not real hard and it would not stay up. My energy level was going down the toilet and I had less "social" desire. My Total T is normal but my Free T is low. I also have borderline high Estradiol and Estrone (Estrogens that men have as well as women) and DHT. In other words, my T is aromatizing into Estrogen. I don't have the outward symptoms (a lot of belly fat, man breasts, etc.), so a doctor would typically not run tests on these. See my posts on the Men's Health board here to see what I am doing for this now.