I know I'm late to this discussion, but I've got to say something about this. In some cultures, cousins do marry each other and no one thinks anything of it. Your ages were also close enough to each other that there is no question of this being a pedophilic kind of thing. In essence, what you really did was to do what millions of 19 year old boys do -- you went and felt up a pretty girl who was available and she didn't like it. Frankly, if she can't forgive you for that, then she has serious problems of her own. You didn't rape her, you didn't abuse her and it's not like you were doing something age inappropriate. While I wouldn't make a habit of doing such things, I'm sure you wouldn't be the first or the last 19 year old boy to do something similar with someone they felt close to and with whom they misinterpreted certain cues.
Bottom line, for most people, this would be one of those embarrassing, funny moments they mention at a party where every laughs about "how silly we all were." You beating yourself up about it three years later, not to mention deciding to commit suicide the next day is the real issue here, not what you did. You are being overly sensitive about something that should not be that big of a deal. Mind you, I'm not trying to minimize your pain, which I'm sure is quite real, but you do need therapy to deal with why something which is embarrassing and somewhat inappropriate but hardly a major issue is causing you so much pain.