For me, it was
fear of the unknown,
fear that I might be making a mistake,
fear that I couldn't support myself or make it on my own
fear that he might actually get well after I leave,
fear of what people would think
fear of what he might do if I left him (would it be my fault??)
fear that I would never be happy or find love again
That's just for starters.
I journaled and journaled about my fears. Sometimes they got so big I coudl barely breath. I went to lots and LOTS of Al Anon meetings, and I talked to recovery friends. Little by little, day by day, I was able to overcome my fears.
Life is good on the other side.
Hugs
Cats