Boss bullying me into staying?

So I applied at my city's police station in the beginning of the year. Never heard anything until recently like less than a week ago. Got the email that my application was accepted and from there things moved kinda quickly.

My coworker found out she was pregnant after trying for 2 years. She goes on maternity leave in Jan. We are kind of the leads for our clinic and our boss depends on us (so keep that in mind).

So my boss is a no BS keep it upfront and honest person. So I go to ger and tell her everything that's been going on and she knew that this police officer thing is something i REALLY want to do. Like I've been wanting to be one since I was 5 but never had the balls to actually follow through but now my chance is here right?! Shes super excited for me helps me with everything and whatnot.

So my boss emails me and goes, well can you wait until after she comes back? Wait, what?! Like this is a first come first serve. They only hire every 2 years type of thing. So if I miss this. I'm missing out until 2021. And even then I'll have to redo everything. The testing, if I make it to the academy, literally EVERYTHING.

Is it wrong of me to tell her no? Because who knows if I'll get another chance like this.