Pressure to use formula when away from baby

I am leaving my baby with my husband next week for a day out with my friends - first time I've done this since he's been born. My friend who hasn't had children couldn't understand why I was worried about feeding that day. I explained i was struggling to pump enough for the 2/3 feeds he may need whilst I was away. She pushed me on why I didn't want to use formula that day instead (which is the reality of what I will have to do). My baby only has formula once at night and it took me a long while to get my head around that. Please don't think this is a formula shaming post - not at all, it's great if it works for mum and baby. But I worked so hard to breastfeed my boy, I cried for weeks and months through the pain, i attended clinics, have sat in the night alone feeding him sometimes hourly through the regression solely as my husband can't do that bit, and then I get asked 'why are you so worried just use formula?'. How do I explain this to my friend? I want her to understand because she's making me feel worse and more anxious about leaving him as I'm questioning myself, why do I have such a issue with it?! I feel annoyed I can't articulate it and I want to be able to. Is it just the emotional part holding me back, is that all I have to explain? Please help me unpick my thoughts so I can get it straight in my head and defend my choice to feed in a confident and assertive way 😔 And again, not shaming formula, I use it!