Preschool

I believe I mentioned in a previous post about Adam getting kicked out of his first preschool for that whole biting issue.
Well, a couple of weeks after that, his new preschool was able to squeeze him into some drop in spots throughout the summer, and then at the end of August, we started his regular schedule going twice a week from 9am to 1pm.
This has been great for me because it gives me a few hours at least twice a week to focus on getting paperwork and billing done during the day instead of putting Adam to bed, kissing hubs and heading off to the office for a late night.
I LOVE this preschool...he has two fabulous teachers who are very montessori style in their teaching.  Every day I am sent home with an art project of some sort, which I love.  There are as many as 11 other kids, so it's big enough that he has interaction with lots of kids, but not so big that he gets lost in the shuffle. 
That being said - we've been going there twice a week now for four months, and he STILL cries when I drop him off.  We've had a few days scattered here and there when I can leave with him just pouting, but most of the time he's crying and calling out "mama".
It kills me.  I can't even tell you how many times I've left that place and climbed into my car and driven away with tears in my eyes.  Not so much anymore, because I know he's in a good place and I know he's fine when I leave, but in the beginning, when they would have to pull him off of me, him screaming at the top of his lungs and clawing at my clothes...and I'd have to hand him over and just bail because the longer I stayed the worse it got....that was awful.
The first month or so, he would start sniffling in the car on the way there, and then the minute I got off the freeway, he would bust out a sob or two.  I would look in the mirror and his big eyes would be huge and full of tears, just quietly falling down his cheeks.  He would softly say "mama, don't go away"...and then I'd have to talk myself out of just turning around and heading home.  Remind myself why we are doing this - but oh my GAWD that was awful. 
He finally stopped doing that, and now the crying doesn't actually start until we are in the door and I've signed him in and we're ready to go.

One of the reasons we wanted to do preschool was for the social aspect.  He is so content to play by himself, that we really wanted him to learn how to share and interact with others.  Also, he's so attached to ME, that we wanted him to learn that it's okay to be away from mom for a while...and when I say I'm coming back, I AM coming back.
I didn't anticipate him having this much trouble to be honest.  I knew at first it would be really hard - but I didn't think he would still have a hard time with it 4 months later.

Have any of your kids struggled with this?  Or do I just have an exceptionally clingy mama's boy on my hands?