It feels like a big problem for you now, but in the big picture, it's not at all. She's only a year and a half. Is it really such a big deal if she holds onto it for a while longer? (As long as you're just giving her water, of course; anything else, and she'll get cavities. If you need to transition, simply water whatever it is down over the course of a couple of weeks. Maybe that, alone, will make the bottle less appealing to her, though she might not quite know why, if you do it gradually enough.)
I think that, probably, what you can do is start introducing another one or couple of comforts *while* still giving her her bottle with a smile. That way she can make the switch more easily. If you give her a super excellent teddy bear that she has to hold with both hands, she might just choose *it* over the bottle. (Try sleeping with it for a few nights, yourself, so it smells like you.) If that isn't what ends up happening, you can, after a couple of weeks of snuggling with her and her new blankie/teddy bear/whatever, gently start to make the switch. Really just focus more love and happiness on her and [fill in the blank] and just act casually bored about the bottle but say, when she asks for it, "oh, yeah. IF you really want this old thing, sure." Don't be bothered if she continues to cling to it for some time. Give the bear/blankie, whatever, special attention, even when she doesn't. Then she'll value it more.
Another approach, in combination with or instead of these is to set a big-girl graduation date. (This might work a little later, depending on how developmentally ready a 1 1/2-year old is for this kind of approach. Can't say I've been there.) Keep talking it up and have it on a calendar that she can see. Maybe it will be Christmas (with lots of distractions to help!) But maybe that will be too stressful a time for her, with Mom being busy with so many tasks. Maybe it will be New Year's Day. Or some other arbitrary day. Tell her that she can trade in the bottle for a great new present - pick it out with her, make sure it's something she really likes, and leave it high up in a prominent place and make it a big deal that she is about to earn a big-girl present. Then, during the event, allow *her* to hand over her bottle. Because whatever way you do this, it will go best if she voluntarily gives it up (thumb-sucking, etc. will be less likely to replace it).
Whatever you do, don't make it a fight. The more stress she experiences at night-time, the more she wants to cling to her favorite night-time comfort!
I'm sure you guys will do great. And no matter what happens, as long as you give her love and cover the essential needs, she will grow up to be a perfectly fine adult :)
L.