Losing erection ability

This question has been here a couple of times, but everyone is unique. So please direct me to my best solution. I'm appreciative of any guidance.

I just turned 24 years old this September. I don't smoke, haven't drank alcohol in over a year, I'm not fat, I've been having sex since 19 years old. I have been with my current gf for almost 3 years. I am still attracted to her physically and mentally.

Up until 2 weeks ago, I was having sex with her with no loss of erection whatsoever! I'd be able to go for about 40 min. or more in various positions, and able to fejaculate or choose to not ejaculate.

In the last 10 days-2 weeks I have been going limp (in comparison to my normal stone-penis). I am still able to somewhat keep it up and also have made her orgasm at least once. But I am no where near what I should be.

In these last 2 weeks, I started working out, studying a new language, and I also spent 1 week not wearing any boxers (I was being lazy). Can any of these things be a factor? Are we doing too many positions, and is my penis naturally inclined to bend now? I used to get hard just b touching her booty, but now my mind doesn't get me hard. I just need some help.

Some of you people with more experience on this planet have jobs, families, etc. But, for me, my sex-life is currently all I really have going for me. I can't lose my sword! Seriously, what is my best hope and reclaiming my status as an above-average lover?