From what you've said, I really don't think it's a third child you are after. You don't talk about a longing to hold a baby or a longing to have the different family dynamic that would come, etc ,etc - just about missing what you have recently lost with your current children, particularly your youngest, child.
Maybe you need to re-examine your decision to go back four days a week. Because you could still have those days that you miss, at least the afternoons after your DD finishes her morning at Nursery. Plus, she doesn't have to go five mornings a week, it's not obligatory! She could go for four mornings and you could have one day all for yourselves...this time will only last till she goes to school so perhaps consider if you can't just make the most of some more time with her? Maybe this would help you examine whether you really want another child or not.
And also, I think go easy on yourself. It is completely natural to feel this sort of problem with coming to terms with things changing as they grow up. I feel it too, that my DS will be in reception in September (at barely four poor mite) and I simply CAN'T BELIEVE that this time has gone - however one way I'm facing it is to look forward to giving him all the help and support he needs throughout school, and I'm intending to be available to help out, PTA, whatever they will let me help with and so that I can be fully involved in this future part of his life.
So I think it's about looking forward as well as back, and accepting that you will feel the way you do. It's such an incredibly precious, tiring, stressful but magical time when kids are pre-school, isn't it. Don't try and brush your feelings away, they are valid.
Good luck feeling your way through this - much sympathy to you x