Substance abuse is pretty common among those with gender dysphoria, and alcohol is often the substance of choice. I fought my own battle with once a week binge drinking when I was in college, and on and off ever since -- until I fully came out to myself and to my wife as transgender, and started dressing female around the house.
All of a sudden my (previously high) blood pressure dropped to 120 over 80, even after reducing the dosage of my meds, and I totally lost the urge to consume alcohol every weekend, and even to have a beer with dinner after a particularly rough day at work. Being able to wear the clothes I longed for also motivated me to diet and exercise. I've lost about ten pounds so far, with another five or ten to go.
YMMV. If dressing, appearance changes, and even hormones do not diminish the need to consume more than a moderate amount of alcohol, (in other words if transitioning is not making your SO happy) then your SO's therapist may need to look for reasons behind the drinking other than gender dysphoria.
Instead of tiptoeing, you should try to find a way to address whatever is really behind your SO's irritability -- whether it is something within your relationship or something from outside.
My own transition is running at a snail's pace, partly to keep my relationship with my wife happy and harmonious, and partly for reasons having to do with health insurance, finance, and my career. I just keep reminding myself that if I made like Caitlyn Jenner, took a six week leave of absence, and came back looking like a pretty girl, in the dangerous and gritty world I work in that would cause a lot of problems of its own.
It is perfectly natural for you to have your own feelings about an SO's transition. You are human, and you are entitled to have feelings and express them. You are not a victim, in the sense that no one set out to intentionally deceive you, but you are taking what in the legal profession is called "collateral damage." You are suffering unintended harm as a result of someone else's necessary action. And you are entitled to feel hurt over it.
So have some chocolate, read a trashy novel or watch a chick flick, or do whatever you like to take care of yourself, and then see if there is a way to lessen your SO's pain.
