x1000 on the "young kids" explanation. You are in the middle of the suck, and it's going to suck for a while, and that's ok. Because being in the middle of the suck means that there is also an end of the suck, and it truly is coming.
My advice:
Allow yourself to feel pissy and frustrated if you're feeling pissy and frustrated. Doesn't mean you're not a good parent, doesn't mean you're not a good partner, just means you're in a tough phase. Tamping down the negative feelings only forces them underground until they get so powerful they erupt on their own and make a mess of your life -- and when you're feeling lousy, I guarantee that the one thing that will make you feel even worse is trying to force yourself into artificial gratitude. I have to go through the anger and frustration and grief before my brain will even consider being open to gratitude.
Work on empathy and perspective. I tend to get aggravated by little things, like someone cuts me off in traffic. So one of the things I do is tell myself all the reasons someone might do that that do not equate to "they're just a total asshole" -- maybe they're on the way to visit someone in the hospital and their head isn't in the game, maybe they just had a breakup and are focused on not bawling in the car, etc. When I can see annoyances as real people with real problems, and not as just some stupid or ill-intentioned "other," things bother me less, and I am happier overall.
When you are in a not-actively-bad frame of mind, this is when you look for things to be grateful for. A cooling breeze in summer, the fact that you can afford to provide your family the lives you want, a good thing that happened, etc. You can't force it, but by practicing that when you're in a receptive state of mind, you can train your brain to think more that way.
Remind yourself that you can have any of those things that you want; you just can't have them all at the same time. You can go travel the world right this minute if you want to; you're not stuck, no one's putting a gun to your head and threatening to shoot if you get on a plane. But you have chosen not to do that, because it would cost you your job and family. You can both quit your jobs and move abroad with the kids and get by on whatever you can make overseas; the tradeoff is stepping away from your career and your home and potentially a longer time to FIRE. I find it incredibly helpful to remind myself that I can do XYZ whenever I want to; I just don't want to, because the tradeoffs would be worse than what I'd gain. That changes the feeling of "I'm stuck in this unfulfilling life" into "I've chosen this life, and if/when this isn't working for me any more, I'll choose something else." It gives you power vs. seeing yourself as a victim of forces beyond your control.
But for the things that are missing, ask yourself: why do you miss them? What is the core feeling that they give you that you don't have in your life now? Then figure out something you can do now that gives you a similar feeling. Sure, you can't hike the Appalachian Trail, but you can take a weekend hike, right? You can't uproot your family and slow travel the world, but you can start dreaming about the places you want to take the kids when they're old enough to appreciate it and saving up for some meaningful family trips while you're in this phase.
The real key to success with this last one: do the same for/with your wife. You can't just announce you're taking off on a weekend camping trip without creating more unhappiness than you have now. But I guarantee you your wife also periodically feels frustrated and overwhelmed and resents that she no longer has time/money for things that were meaningful to her. If you both do the thinking and talking, you can trade off free time to give the other the needed break -- maybe you go hiking one weekend while she minds the store, then the next weekend she goes away with her girlfriends to some cute nearby town. When our kids were little, DH and I swapped weekend mornings -- I got Saturday morning to do whatever I needed, he got Sunday. I used mine to sleep (nursing, ugh), he used his to go play golf. It truly saved our sanity sometimes.
Finally: carve out one-on-one time to spend with your wife. Work and kids are very loud and demanding for the both of you; your need for a connection with each other is equally important but easily drowned out in the daily rush. Just as you plan time and $$ for your individual needs, plan time and money to be a couple. A lack of meaningful human connection can contribute a surprising amount of dissatisfaction and frustration; making time to reconnect with your person can take the edge off things -- even if you spend it just bitching about your frustrations, at least you're not bitching alone (or to an internet full of strangers ;-)).
