My best friend is suicidal and I'm in love with them, madly.
Are you in love because they're suicidal?
Why else would I fall for someone who wants to die?
It is okay to have deep feelings for someone. You should be aware though of how your friend feels about you.
She tells me it's fine if I'm sad and that she'll be sad without me.
Your friend is suicidal and needs help and you should help them get what they need. It can be appealing, but it's not realistic and loving them means helping them.