I'm so sorry about your friend, and that you're having a bad time. Welcome to our little safe zone, I hope you can find some healing.
how does an ex-witness ever have a 'normal' relationship, where you're not constantly fighting subconscious guilt for being with the person you love?
I hope that you and your partner can talk about what you need to do to put it behind you, and that your partner can understand that it will take longer than they think it should.