Feeling like a placeholder

Me and my girlfriend have been in a long distance relationship for over two years now. Some months ago she met this guy on the internet and told me she wasn't sure if she was developing feelings for him, but afterwards told me she wouldn't break up with me because our relationship is really special to her. I went to visit her this weekend and it was really great, but the other guy went to visit her this weekend (she told me he was visiting her this weekend months ago) and I feel really ignored by her these last few days. I feel really stupid and that I'm just a placeholder while she makes up her mind on whether she likes this guy more or not. I've been having anxiety attacks every other night over this and have cried myself to sleep several nights thinking that I'm just not good enough even after these years together.