I'm wonder what have you done to deal with your depression? Have you tried meds, counseling, ect? The reason I bring this up is because with the lack of "in love" feelings on your part I feel for her if you tried to take her away from her boyfriend because you yourself are mono and are intimidated by him. It screams of you stripping her of what satisfies her needs to make things more comfortable you. Depression affect not only the depressed. It affects those that love them. Depression can be a very selfish disease. Similar to addiction. It can strip and drain loved ones as the depressed one takes and takes physically and emotionally without giving anything back to replenish that loved one.
I would say to try to view this from her side. She's your friend. You've got her presence in your life. Maybe you should weigh if it's really fair for her to be unfufilled because of your fears. Maybe you should dive into working through you depression first before approaching her with life changes in her life for you. My fiance has gone through emotional detatchment and he is extremely mono. He knows that I am poly and he sees that I have needs that go beyond our relationship. All he wants is my happiness and while it was a major struggle for him to open up to my boyfriend and also to get over feelings of possession, inadequacy, and fears of me just leaving him, we have found a balance for us. And he's gotten a great friend out of it. They are completely opposite. In "real life" they would have never been friends, but now they have a common denominator... Me. For me and my happiness they dropped their guard and talked through their issues with each other. Not something that my fiance normally does.
basically what I'm trying to get at is, while yes it's easier to ask her to change her life to suit you, don't you think you should first try to adjust things on your side to suit the life that she made for herself with complete honesty? Please look up xeromag and read. Read it all and then reflect and contemplate before making a decision.