As trite as it sounds, "This, too, shall pass."
We went through the same thing with my daughter when she first started attending daycare.
You may want to look at how the daycare takes care of it's facility. Do they wash the toys everyday? Do they make the kids wash their hands all the time? Do they have a strict policy of sending sick kids home? Are the kids noses running all the time?
The only way you can assure yourself of these things is to visit the daycare during operating hours.
Also, ask other parents at the daycare if they're having the same problem.
I eventually stopped getting sick mainly because we switched to a new facility. It makes a difference.
The other option is to also take really, really good care of yourself. It's tough being a single mom (my sister was one and I saw her run herself ragged). Try to take some time for yourself. Hopefully you have a supportive family that can help.
Good luck.