Another option, very similar to mutual masturbation, would be to ask your girlfriend to use your hand to simulate herself. You let your hand completely relax, she puts her hand over yours, and she uses her hand to control the pressure, speed, etc. Then not only are you seeing exactly what she likes, but you're feeling how to do it, as well, to give you a better idea of what sort of pressure and speed she prefers, when she likes to go faster or slower, and even specific places she likes to touch or give more attention to. It can definitely be a little awkward and clunky to get used to you keeping your hand relaxed and letting her guide you and for her to guide a hand that isn't her own, but you might both find it intimate and sexy (and don't be afraid to laugh when it is awkward) - and have it pay off in the longer run, too.
Also - and I know this can be hard in the heat of the moment - but practice having some mindfulness and extra awareness the next few times you're intimate with her so that you can practice more being slower and gentler than you're used to. With more practice, it'll come more naturally rather than being something you have to stop and specifically force yourself to do.
(And yeah, HRT can definitely impact sensitivity - and impact it in different ways depending on the dose and length of time she's been on it! Not only physically, but HRT can cause mental differences in how we perceive ourselves, too, and since the mind is very wrapped up in sex, how we perceive ourselves can also change how we prefer to be touched and what we enjoy.)