Want to break up but scared he will self harm

I have been with my partner for 18 years, married for almost 10 years. He is over 15 years older than me, I'm in my mid 30s. I feel I can't be with him any longer for numerous reasons and my biggest concern is his wellbeing. He suffers from depression and has attempted to commit suicide in the past (before I met him). He loves me and he knows our marriage hasn't been perfect but he doesn't know how I feel about our marriage and that I want to separate. I am terrified of bringing it up because when we've had arguments in the past he has self harmed. I feel like if I bring up the conversation his world as he knows it will shatter and his depression will get significantly worse to the point where he could try committing suicide again. If anything happened to him I could never forgive myself. I feel trapped.