She has no idea I am a drinker

My girlfriend and I have been together for around six months, and she doesn’t know that I drink alcohol. She has a serious problem with it which is why I lied, but it is starting to bother me because I am worried that she may break up with me immediately if she finds out. It’s not as if I want to lie, but there was no other way because I love her a lot and didn’t want her to not be with me for that one reason alone. I am constantly afraid that she may find out about this from someone else, and it causes me a lot of stress. At the same time, if I come out and tell her the truth, she may assume that everything else I have told her is a lie. What do I do? I can’t lose her just because I have withheld this tiny piece of information.
You have no choice but to tell her the truth, not because this is bothers you but because no relationship that purports to be based on trust and mutual respect should be built on lies. This is clearly an important issue for her, so dismissing it as a tiny piece of information only absolves you and blames her. If she thinks everything else you have told her is a lie, that may be a natural reaction and it will require you to rebuild trust. The longer this lie goes on, the harder it becomes for you to come clean, which is why you should be honest about it as soon as possible. There is always the possibility that she may accept your explanation and understand why you weren’t truthful, based on how strong your relationship is. Either way, you won’t know until you speak up.

It feels as if my relationship with my sister is shallow because she is never there when I need her. How do I get her to understand how I feel?
You may both need to adjust your individual expectations.