I hear you! My DP is also forces and while I obviously don't like him being away I can cope. What I really struggle with is the unexpected changes of plan. Hate it hate it hate it. I can deal with most things but with no time to prepare or wrap my head around what's happening next I feel awful. It hits me much harder.
Take this year - I knew he'd be away for 4 or 5 months and I knew roughly when that would be. Ok, well I'll pull my big girl pants on and get on with it. Just this week I had only a days notice that he'd be uncontactable for 5 days (I wasn't expecting that to happen again on this deployment)... I cried like a baby! Just one tiny example of the countless occasions our plans have been completely erased because he got the dreaded phone call.
So yes, I absolutely know where you're coming from. You feel utterly helpless at the hands of the military. You don't count to them, they don't care what's going on for you, what plans they spoil, how hard your life is (and nor should they, of course they don't!). And of course we knew it would be like this, we knew his job would always come first, but the reality is so much harder than I could have ever imagined and I've been living this life for years. And of course I stick a brave face on and never really complain to anyone because as you say - unless they've been in our position, how can anyone really understand? People who know he's away are generally very kind and sympathetic, but they don't, can't know. We don't live anywhere near his base so I don't have a network of wives who are experiencing exactly the same.
So yes, it is completely understandable to be thoroughly gutted about your DH's unexpected departure. No point in saying "chin up" because you'll be doing this anyway. I have no advice at all other than do whatever it takes to get through, for the children AND you. Be kind to yourself, this is rough and it's alright to feel sorry for yourself. No doubt you've done this before and you know you'll be in the swing of it soon enough.
I really feel for you. Take it one day at a time, count the weeks not the days, cry if you need to at first. Wait to make a decision over your holiday until that raw edge of the goodbye has faded a bit. You'll know what's for the best when the dust has settled.