Need Advice (Relationship) Long post

Hi. I am new here. Looking for advice. Is it fair to do what I feel is best for me even if it will hurt someone else? I have anxiety and bi polar. I have been with "Sam" for 9 years. He is a very negative person. This does affect my moods. I have spoken to him about this. He has tried to change but old habits..For better part of 9 years I have felt unloved and unwanted. 4 months ago I met "R" who is the complete opposit of Sam. At first I though I was having a bi polar mood swing and that my feelings for "R" werent real. I am sure they are. It was only after Sam found out about "R" did he suddenly start showing me that he loves me. I want to leave Sam. Not for R exactly but for all the times he made me feel worthless and all the negativity. I am 45 and want a future where I will be happy. Is it fair for me to leave him even it means he will be hurt? Thank you for anyone who replies