I can totally understand alot about what you are saying and experiencing. Was your SO ever more "into" sex? Mine was and then totally shut down.
If I try to talk about it to my wife, it just spirals downward so I know why you avoided talking to her and thus were then labelled "crabby". You cannot get down on yourself though. This is all about her sexual receptiveness to you. I have the same issue. I do not feel unattractive, but I feel like I do not "do it" for her.
It does get lonely though and frustrating. Since I am married, I absolutely will not go elsewhere for sex so I am stuck either w/out sex (or rarely) or I have to leave the relationship entirely.
It's as though the sexual aspect of the realtionship has run it's course and now all that's left is the friendship.
I feel for you. And often I do not know that the low drive person even really understands the depth of how this effects the relationship. You are not alone, if that helps any.