My daughter is 19 months and she started preschool in September too. She goes for 3 hours a day, 5 days a week. Just like your daughter she was fine for the 1st 2 weeks or so (well, actually she did cry the first 2 days, but then after that no crying for a couple of weeks). Then the crying started-she'd cry when we'd approach the school and then start crying when they went out on the playground before pickup (cause she knew that was when she got picked up and saw other kids leaving). After about 2 weeks, the crying dropped to only at dropoff (no more crying at pickup) and then about 1.5 weeks ago she stopped crying at dropoff too! Well, today my husband came with us so that changed her routine and she cried this morning, but I think she'll be fine again tomorrow.
I know exactly how you feel about walking away from her while she's crying cause I felt the same way everyday. There were times I really fought with myself about just taking her out of school and waiting till she's older, but then I thought that if I just stick with it then she'll get used to going and the crying will stop. Her teacher (who I really like) assured me that she stops crying as soon as she gets in the class and loves being with the other kids (though she did add that she is still she around the teacher, and prefers to talk to 1 of the assistant teachers most). Working in a daycare/preschool before my daughter was born convinced me that her teacher was telling the truth because we had plenty of kids who cried at dropoff but were very happy all day long.
What's probably making the transition take a little long for your daughter is that it's 2 days a week. I found when I worked in the daycare that the kids who came for morning preschool 2 days a week generally took longer to stop crying than those who were in the daycare part, since the daycare kids were there everyday so it became more routine for them to go everyday rather than just 2 days a week. Same with this other preschool we looked at-they had a 3 days/week or 5 days/week program and the director told us that the kids that go everyday stop crying sooner. So I would give it more time and I'm sure she'll stop crying and have fun.
And since I'm posting this after just reading the questions about why send your kid to preschool, I'll give my reasons. I am a SAHM so i could just keep my daughter home with me all day, but I chose to send her to preschool because I wanted her to be able to interact with other kids and learn those very important social skills early on in life. When she's home with me, that's it, just me. I took her to Gymboree classes before she started school and she'd get to play with other kids there, but I felt she needed more than 45 minutes/week. Plus, since she's home with only me, she's very attached to me, which of course is a great thing, but I want her to learn that when I'm not around she'll still be okay. My husband is in medical school and therefore wasn't home much since she was about 4 months old so she is really dependent on me and I didn't think that was really in her best interest long-term. I know 18 months sounds very young, and I was initially thinking I'd send her at 2, but this school takes them at 18 months. She was initially wait-listed (preschool in DC is crazy!) and we thought she'd be starting in January (they have 2 start dates, Sept. or Jan.) but in July they called us with an opening for the fall so we sent her. So far I'm really happy with how she's grown since she started school. I'm not concerned at all about what she learns academically, but will say that her speech has improved dramatically. When she started school in mid-Sept she was saying 12 words and now 6 weeks later she's saying over 50 words. She loves other kids and runs to play with them now, she does so many things by herself that she had no interest in before, and she's sleeping better on her own with I honestly believe is a result of learning that she's okay to do things without mommy. And we still get tons of cuddle time everyday-she's with me from 6am-8am, then I pick her up at 11:30am and she's all mine till she goes to bed around 7-which is now so I better run!