How to improve my performance

Both my fiance (28FLL) and myself (28MHL) are sexually frustrated and for different reasons. She is low libido and I am high libido. We have sex once or twice a month. First she never initiates, or shows any desire for me. Being that we only have sex a couple times a month it makes it hard for me to last long which always leaves her disappointed and unsatisfied, my self included. My frustration comes from being horny all the time and wanting to have more sex. If we had more sex I could control it better, but we don’t , and she expects me to be ready to go and last for ever when we sporadically do have sex. If I jerk off before sex I’m usually able to last long enough. But I never know when we will be doing it so I can’t prepare properly. I’m so tired of disappointing her and feeling this way. I can’t stress enough to her that if we had more sex it would be better , but she just doesn’t get it. I’m at a loss here , it’s driving us apart. How can I improve myself and last longer?