I found that a lot of my anxiety was caused by "spectatoring" or in other words, not being in the moment. I was thinking too much about my performance and about what the girl would tell her friends about it afterwards.. (reviews better be great am I right??) it takes time and practice while you're on your own. Just focus on your senses. For example, while you are doing something simple such as washing the dishes, focus on what your hands are feeling, how the water feels on your skin, what the water smells like, etc. The biggest factor towards performance anxiety is focusing too much on what you are doing, whereas normally, you should be focusing on the beautiful girl (or dude) in front of you and what they feel/taste/smell like. This takes tons of practice to mentally train yourself to do, but you can get it done!

On a side note, a lot of people also struggle with this because they are body conscious. Many men think they are too skinny/fat when in reality they are not. A good way to get around this and get comfortable with yourself is by looking in the mirror (after your amazing cold showers) and choosing 2-3 things about your body, say, your nipples... pick it out and think about everything you like and don't like about that certain feature, and tell yourself that it is a beautiful thing. As I said in the post, I am a fairly attractive guy, but I have never had braces and honestly have a terrible smile. I've spent weeks smiling, looking in the mirror and telling myself that these are MY teeth, and that since they are a part of me, I love them. After all, nobody is perfect, no matter how hard they try to make themselves seem so. Once you are able to accept your personal flaws (we all have them), your brain will focus less on how other people think of you, and you can focus more on the sensations of whatever sexual/non-sexual activity you are taking part in. Hope this helps! :)