You are absolutely correct, you are not alone. There are several of us ladies (and men) in the same situation you are in.
If I understand your post correctly...the first two years of your marriage you guys had a healthy sex life. Is that correct? This wasn't just something that started shortly after you got married right? Were there any major events that happened around the time that the sex began declining? Does he have any health issues? Major work issues?
I also hate to bring this up...but I must, are you POSITIVE there isn't another woman in the picture?