I've had erection problems--I'm in my 40s; and I agree with the advice above about fitness and especially getting more sleep. But since you are fit and in your 30's let me chime in with another possible cause.....
In my case, embarrasing to admit, the other causal factor was porn--- internet porn, which, turns out, is like porn on 'crack'.
I'm no puritan, I enjoyed porn mags and dvds growing up, with no erection problems, so at first I didn't accept that intrnet porn was any different....
But then read some research showing that the instantly accessible novelty of having all those tabs open can really do a number on the chemistry in the male brain. TEDx talk: 'the Great Porn Experiment' explained how dopamine 'saturation' in the brain results from the super-increased novelty of internet porn; this in turn can result in Erectile Dysfunction--which is now quite common among 20-30yr old guys because internet porn affects the brain differently than playboy magazine and dvds did for us older gents. The dopamine saturation can also disrupt sleep and 'normal' pleasure response in non-sexual activities, leading to increased depression.
The website 'yourbrainonporn.com' explains this new phenomenon in detail without any moralistic judgementalism.
Solution? Going 'cold turkey' --a complete avoidance of internet porn, to let the brain's dopamine receptors reset to normal levels.... took me 8wks, then my normal erections (like 5 yrs ago) returned.
I kept thinking I could have a lil bit of pornhub, but for me at least, even a little bit changed my heart rate and sleep pattern (and erection strength for intercorse), so i had to go completely off it.
There's an online site where guys talk about their efforts to deal with this: yourbrainrebalanced.com.
Hopefully this is not the issue for you; but I post this in case it is for other guys.
I've read a bit about this--does it actually work though? I'm a little bit skeptical. I guess it's worth a try though. Gonna be difficult to say the least though hahah

