If there is no more light in a relationship only darkness and bitterness - there is no point to go on, hoping things will be the same as before. No they won't. It is a biggest self deception when you believe you can reverse the relationship to the point when it was beautiful. However both of you need to learn a lesson from each other. His darkness might be something that is teaching you the lesson. Maybe there is something within you that you hate so much and you don't want to see, so you hide it deep inside and pretend it doesn't exist, but then it gets projected on a partner.

I once made a choice (subconsciously) that led to the end of beautiful relationship with someone I thought was the love of my life. The formal reason was responsibility he had to take. The real reason, however, was within me - my own responsibility issues.

Everybody around is your reflection, but a partner reflects us more than others do, so the best we can do is to look for reasons and answers within ourselves.