My immediate thought is you're feeling guilty for perhaps promising more than you can reasonably deliver. Obviously you didn't want her to abort your grandchild, so naturally you told her you would help. But now you see what that truly means.
Is there any way you can help other than being right there where she is, away from your husband? Perhaps, for example, you could provide financial help, so she could hire a babysitter and/or even put the little one in some day care at some point.
Is there any way she could move nearer to you and your husband? I know the military provides housing, but.....I assume your husband refuses to move to be where she is, and frankly, I can understand his position, if he does.
My other thought is you made a committment to your husband before you made any commitment to your daughter. I honestly don't think you'd be doing him right by leaving him. I do know some grandparents actually take over the raising of their grandchildren, but I don't know your and your husband's health and desire to take on such a big responsibility.
Have you talked to your daughter about your dilemma? I suggest you not mention leaving your husband as an option and see what she says. She might try to play on your guilt, alas.
Keep us informed. I can tell you are a caring person.