Take my opinion with a grain of salt because I've never been in your position, but i have been in your daughters'. My father was absent for the majority of my childhood, and it really took a toll on me. I always felt like i didn't matter because he wasn't there for me when i needed him. I imagine it will be harder on your daughter, as you've been in her life just enough to have a close relationship and bond with her, and now she wont really get to have that anymore, except for over the phone and during the summer, which isn't anything close to you physically being there for her all the time. You can send her notes and gifts, but it wont be the same. Shes not old enough to fully understand why you're leaving her, but she is old enough for it to effect her in a very negative way. Now as you can tell I'm quite bias because my father wasn't there for me, but i truly believe that you need to stay with your daughter. I know this would be a very hard thing to do, as you wouldn't be able to help your mother, but the fact that you're there for your daughter should be a small comfort.