I am not so sexually experienced, but I've had sex for sure.

One thing I sometimes have a problem with is the actual penetration. Missionary is usually okay, but doggy style isn't!
I get a bit nervous standing there behind her and putting it in. For some reason I have a bit of a phobia for not fitting the hole and managing to slide it in correctly. This may sound crazy, but I'm not joking. These difficulties can result in me losing the erection, which makes me even more anxious and thus unable to get the erection back. :/

How can I get better at this? I don't have a stable partner and I do not want to look bad in front of a new one.