Worried about leaving my job to go on mat leave

They are currently recruiting for my cover. I’ll be off for 13 months. I’m nervous that they will replace me even though they think highly of me 😢 how can I get these worrying thoughts out of my head. The potential candidate has already been asking what will happen after the 12 months is up. I think I’m nervous because last time I went on mat leave, my cover ended up being my manager and I don’t want it to happen again, I don’t want to miss out on those opportunities and be left behind just because I’m on mat leave.