When my kids were that age I was in a similar head space.
So little bounce in me that the slightest extra difficulty felt enormous (like a small perceived criticism,).
I had lost all my sparkle by pouring from an empty cup for far far too long.
Every step I took felt leaden, every smile on my face was pinned on with an act of will... And I didn't like feeling that way, it often sucked all the joy out of those little fun moments kids give you
In my case it was the natural demands of parenting children of that age + very little practical support or help (for various unavoidable reasons) so I was doing almost everything domestic night and day + doing a degree... So compared to you - different but similar iyswim.
One effect on me was that I entirely lost my sense of humour, zero humour, but that was the glue of my relationship with DH, so who was I without it, and what were we as a couple!?
I dragged my sorry carcass through one day after another. But luckily, like you, I had a good DH who didn't fade out stage left when the going got tough. He stuck by us, and battled along with me, going through the same things just from a different perspective.
Two things helped.
We didn't get into a toxic one up manship of who had it worst. It is really tempting when trying to elicit sympathy or support to emphasise the rough time you're having and claim the status of most knackered and fed up... We talked and made a pact to not do that. Instead we asked if the other was ok and did they need anything... It was a conscious discussed choice to do this.
The result was that we both felt we had each others backs, so the urge to complain was minimised, instead it encouraged us to put our best foot forward because we saw each other making the effort for the other. The slog was not going unnoticed.
The second was we kept talking about what we valued in life 'before' and how it would be again, and made tweaks in that direction where we could if possible... I.e. at one point we were so under siege that we were barely making eye contact, ships in the night X 100... We realised and started making the effort to make eye contact and be pleased to see each other...
You know you're doing real in the trenches work, when you're fighting to maintain basic connection never mind sex life!!
But we did, and it slowly improved and we slowly got better.
I had CBT which helped me reverse the utterly flat dead inside feeling which the relentless slog has created.
We even managed to get the sex spark back, which was a pure act of love as it has been so far on the back burner it had become weird to flirt with each other... But we did.
Now we're ten years on, the kids are fantastic and we are still going strong... Wasn't easy but so worth it, we're a team.