Your setting yourself up for failure by how your thinking. Relax. Sex should be fun and passionate not a chore. Erections come and go all the time its no biggie. It just gives time for other ways of pleasing, I mean your penis isn't the only instrument that could please. Let it come and go. Feel comfortable with your sexuality, and being close to your woman. Instead of thinking just feel. Experience everything. Take note of all your senses. Imagine making love is like a dance, where the music is the passion between you and your lady, the instruments- be they kisses, careses, holding, appreciating, moans, hair, all choreographing into a beautiful symphony.
Don't compare yourself to real or imaginary lovers. If you worry about what she thinks communicate with her how to tailor to her needs. There are a lot of supplements on the market that will help you by increasing desire, and blood flow. None can compare to how you use your mind.
I believe you are attaching unpleasant emotions and memories to a future condition. You get what you expect, so busy your mind with a flawless performance. Don't make it such a life or death situation enjoy it.