I think I felt like you do now. I had a very clingy little one. What helped me was going to early childhood family education (ECFE) classes - in our area they have a time where the parents leave the room for a bit. This was DS's intro to being left by mom (of course he hated it at first). I couldn't wait for this class for at least 20 minutes of freedom! I made several friends in these groups and DS got to know a lot of kids his age. When the preschool decision came around, there were 2 kids from DS's ECFE classes going to the same preschool. This helped. Our ECFE classes were also age-based differently than our schools, so DS met kids that would be in a class ahead of him too.
I agree with chris - your mom just might be trying to help with her comments. I know my mom told me several times that I needed more me time.
As for preschool readiness - I don't think there's any such thing. Around here, preschool is seen as getting the kids used to being around other kids and paying attention to a teacher. Most people I know do 2 years, but many families do 1 year or no preschool. If you really have your heart set on a school when your DD gets older, you can call the kindergarten admin and ask what they expect and how many kids have preschool experience.
For me, preschool readiness was basically mom is sooooo ready for a bit of alone time. My DS started at 3 with a january birthday. He had a couple girls in the class who turned 3 after preschool started. One was very small, and she has a pretty quiet personality, and she did fine (she and DS have always been buds because neither of them chooses to participate in the songs.)
We lucked out with the preschool choice though - we had a reference from a mom of HG kid who liked it there, and also we had no choice as to pt vs ft - there's only pt in our small city.
