In this discussion, I'm going to talk about developing 'sexual triggers' to help you encourage blood flow to your dick so you can get hard. I'm not a sex therapist. I have a lot of experience though - I've had sex in public, in front of friends, outside, in the shower, in not-so-ideal places, etc.
Sex is nearly all a psychological process. That's why anxiety can get it way. If you aren't generally relaxed and confident- you ain't gonna perform too well.
There's a long list of stuff that can negatively affect your performance (we'll touch on these in detail another time).
Of the top of my head, things like - alcohol, drugs, general performance anxiety, she's too hot, she's not hot enough, dehydration, too much water, my legs hurt from the gym, tight underwear, I gotta pee, I gotta fart, I gotta poop, the condom is feeling tight, this position is weird, I blew my load either in the day and temperature can all potentially affect your performance and prevent you from getting a strong erection (or even getting hard at all).
If you have a big dick or been working on it via 'Get Hung', you're going to need even more blood in your dick to get and maintain a nice strong erection.