Hard time enjoying time when SS is away

I don’t know if this is a stepparent thing, an anxiety thing, or if anyone else experiences this.

My stepson spends the summer with his mom, along with every other winter break and spring break.

Every time he leaves, I tell myself I’m going to enjoy the quieter house and the break. And I do to an extent but almost can’t fully relax because my brain keeps reminding me, “It’s only temporary.”
Instead of enjoying the present, I find myself mentally preparing for when he comes back. I’ll start ruminating, anticipating problems that haven’t even happened yet, or thinking about things that might be stressful.

I’ve even noticed I avoid talking to him when my husband is on the phone with him while he’s away because it feels easier to stay mentally “off duty.” Hearing from him pulls me right back into thinking about everything, and then I have a harder time relaxing again.

I wish I could just enjoy the time while he’s gone without feeling like im keeping track of how much time is left or mentally rehearsing future stress.

Can anyone else relate to this? If so, have you found anything that helps you stay in the present instead of anticipating their return the whole time?