Maybe try to make it gf-centered non-phallic sex to the extent possible, giving her orgasms (manual, oral, whatever she likes) as a rule right at the beginning. Then your erection would be icing on the cake. erections are only a must for reproduction. A woman can enjoy sex without them. Luckily. Sex is not tied with reproduction. For recreational (non-procreative) sex, erections are sure very nice, but not a must. That should be your mindset. Because you in reality have two problems, not one: 1) your performance anxiety and 2) her believing that 1) is due to something being wrong with her. If you make sure to shower her with affection and orgasms, you would take care of problem 2) and it just might in turn help you with problem 1).

Good luck!