There are a few things here...
I'm curious about your attachment style with your daughter. Are you protective, cautious, anxious or in "the way" a lot? I don't mean any of that negatively BTW! Children are wonderful emotion sponges, so she will feel it if you are unsure. Children must rely on their care-givers for cues that things are going to be fine. It's why when my daughter falls over and scrapes her-self I just pick her up and we carry on. No drama, no fuss.
The reason I ask is because you sound anxious about sending her away and a little over-attached. We as parents have to let go, let them experience things and thrive. I say that because you seem to think her being at home with you (and constantly being happy) is better than her feeling discomfort, working through it, meeting new children and then learning to deal with emotions in the future.
Her telling you she misses you etc is her way of shaping you to feel guilty. It's what children (ALL children) do.
Just some ideas:
Validate the fact she misses you AND remind her that she has some other children to play with at pre-school too.
Reinforce her going to school. What incentive does she have at the moment? Anxiety, sadness? Heap on the praise when she gets home. Make cakes, paint, take her out to play football... ANYTHING that reinforces school = good thing.
At the same time, I can feel your pain. Ours is only 2 and we have this to come. Though she can't wait to get rid of us! We took her to family recently (we don't see them often) and wow! She told us to go out! Just keep reminding yourself that an unhealthy attachment increases the chances of her having MAJOR psychological issues later. Sending her to meet new people is a way of getting her used to socialising with "the tribe" - the longer you wait, the harder it will be. Unless you want to have her at home until she is 40 (and if you do, that isn't healthy).