If I had a dollar for every time someone has said "I never planned to
co-sleep but…"  I hear you. First off, don't beat yourself up over
that decision. It was wise at the time and you just want a change now.
Doesn't take away the fact that it was the right thing for your family
at the time.

I can
give you the why: she turned 18 months old. This is BY FAR the biggest
change your baby has gone through in her short life. There is a huge
burst in "working memory" (think of working memory like the computer's
RAM; it's what you can hold in mind at one time).  That underlies a
bunch of changes that start happening during this time and all of it
can kick sleeping to hell. Here are some of the details from the
book:

FROM BEDTIMING: "On the bright side, the period beginning at 17-18
months is a glorious phase of development. You will notice massive
changes as your child moves beyond infancy into what can properly be
called early childhood. …language development begins to skyrocket as
children learn to use two-part “sentences” that can describe exactly
what they’re thinking and what they want (Mama up; cookie NOW; NO
beans!). They can also begin to understand social roles, conflicting
goals, and other kinds of symbolic relationships, including simple
rules and ideas such as cleanliness. These are huge cognitive changes,
and indeed the 18-month transition is considered a major stage shift in
many theories of cognitive development. But more than that, it is a
period of emotional reorganization (or total chaos, if you're trying
to get the child to JUST. SLEEP. NOW). Children now begin to recognize
themselves as beings in a truly social world made up of other people.
They see themselves as individuals, with goals and wishes, and they see
other people as individuals, also with goals and wishes. The
realization that their wishes might just conflict with yours is an
enormous challenge to what was previously a strong and super-hero sense
of self. Now they’re not so sure whose wishes are going to prevail. As
a result, conflicts grow in number and intensity, the word “No!” makes
its debut in earnest (yes, they've used it before, but NOTHING like the
force with which they do now), and toddlers try to establish a sense of
security, balanced with a sense of independence, by commandeering their
parents on some occasions, defying them on others, but always looking
for signs of social approval, to reassure them that they are still
members in good standing in the “club” of social connectedness. This is
why we call this the period of "social negotiation."  Of course, these
emotional changes can be difficult, but they are also touching and
heart-warming. Your child is a real, independent little person now.
With vulnerabilities and insecurities to be sure, but also with a new
capacity for understanding and intimacy, and a kind of tenderness and
love that we recognize as being very much like our own. Our babies are
no longer tiny aliens: they are now complex little social beings, with
distinct personalities, needs, and ways of doing things.
    What aspects of this period make it especially troublesome for
sleep training? Separation is more painful now than it has been since
the age of 8–11 months, because it is interpreted according to your
baby’s sense of your goals and agendas. If you are somewhere else, you
could be intentionally ignoring her, perhaps because you have better
things to do. Physical separation means psychological separation, and
that is not a comforting option! Research has shown the highest peak in
separation distress at exactly 18 months. Rather than being alone, the
18-month-old now wants to be with you…sometimes as much as possible.
    So what happens when you explain that it’s time to go to sleep? You
kiss your baby goodnight and tiptoe out the room. You probably won’t
have to wait long before the first howling protest: “Nooooo! Come
back!!!!  No sleep!!!!  Want kiss! Want kiss! Want more! Get Daddy!
Daddy come! Mommy come! Want Mommy, not Daddy. Want Daddy, not Mommy!” And on and on. This is precisely the
social negotiation that you can expect at this age. Your child is using
his collection of words to tell you what he wants, in as many ways as
he can, knowing full well he’s opposing your wishes. His anxiety
escalates as he realizes that you are becoming impatient and irritated.
He therefore needs even more reassurance, making him increasingly
determined not to be alone, as the struggle continues. Yet each time he
tries another tactic, he feels like he’s losing a little more of you.
He is a little more in danger of being tossed out of the “club” of
social closeness, or at the very least having his membership privileges
revoked. And he knows it, but he can’t stop, because to give in, to
stop protesting, would mean accepting your absence and giving up his
ability to enlist your care. If you disappear now, he’s alone, really
alone. And what’s more, he is not very good at accepting defeat these
days. Because not getting his way means a loss of his sense of power
and effectiveness in the world in general and the social world in
particular. And that feeling of helplessness is devastating.
    The emotional changes that rapidly cascade from about 18 to 21
months are likely to make sleep training a dramatic, traumatic, and
ultimately ineffective effort. And the reason for this is simple.
During the period of social negotiation your baby is attempting to
balance her needs for security, a sense of acceptance, and confidence
in her parents’ love on the one hand, with her need for independence
and a sense of competence and accomplishment on the other. Your baby’s
attention is focused on conflicting goals and wishes. Frustration is
met with tears, anger, and very often the first temper tantrums. For
your child to give in and go to sleep would mean admitting defeat,
which means relinquishing her sense of being important and effective.
This is tantamount to having her newly established sense of power
snatched away in one fell swoop. Indeed, sleep training at this age is
bound to be hampered by two interlocking issues: your toddler’s fear of
separation and his determination to hold his own in conflict
situations. Because of both issues, sleep training will be a
hard-fought battle for the 17- to 21-month-old child."  END QUOTE.

So, my (probably horrifying for you) advice: Wait it out. You’ve made
parenting decisions that suited your and your baby’s needs and they’ve
worked so far. Just hold out a little longer before trying to make any
changes. By 22 months or so, you can try any of the top 5 or so sleep
training methods that best matches your parenting style and your child's personality. At that time, your girl  will probably respond quickly to the
same efforts that seem so doomed right now.