When everyone loves your Mat Cover

I feel a bit stupid but I have had an absolute meltdown this week about work and not for the reason I thought it would (i.e leaving my baby). The person who is my Mat Cover obviously leaves when I return but everyone loves them. They have been amazing it seems at my job and I keep seeing emails flying around about how much they will miss them and what a great job they have done. It is like they are all devastated they are leaving and I don't feel like I am exaggerating!!! It's made me feel so rubbish and down and now I'm freaking out about returning. They probably are better, they are full time, in their 20s with no kids. I am going back part time, in my 30s and have 2 kids and probably don't have the same drive or brain capacity. I feel like everyone is like why does she have to come back. I don't know how to stop feeling so insecure about it all. I feel like I have lost my identity, confidence and myself. Then I freak out that I will miss my baby and I wish I didn't have to go back. I feel crazy.