what makes your partner comfortable enough to cum with you and do you have any tips or ideas?

I enjoy having sex with my girlfriend but the issue that I keep having and probably why we haven’t had sex in a very long time is because I take soo long to cum and by the time I do cum, she’s either not interested anymore or just lays there til I finish. I want to be able to cum easier but I have a mental anxiety that makes it hard to cum because I can feel myself anout to orgasm but then it goes away. I do masturbate maybe 2-3/weekly and stopped for a month to see if that would ease my mind but I still couldn’t cum. I want to be able to cum with her whether its full on sex or quickies before we go somewhere or we can sneak off to have a quickie. I recently bought lube and put it in her underwear drawer and she’s seen it but I’m nervous about asking her if maybe giving me a handjob from time to time would help me feel more comfortable with her and feeling her touch but I don’t think she would be comfortable or willing to give it a try and see if it helps me feel better about cumming if I’m able to give her different ideas about how I masturbate then we can ease into mutual masturbation to become comfortable with each other at our most intimate and vulnerable moment.