Why do I always blame myself for things I didn't do in the relationship?

Yes, it is a mystery that why are we doing or claiming things that put us down. Here, i think it could be that you fear loosing your partner and also have serious doubts about yourself. Also it can be apprehended that your beilef that your insecurities may upset your partner and that might lead to rejection from your partner's side. The fear of rejection is one of the greatest fear we all have. Identifying what is the insecurity might help us a lot and also giving a try to discuss the problem with someone might also bring some releif and insights to the problem.