I have the same age gap as you - twice! - so I'm trying to think back 5 (or even 21) months and rattle off some things that helped me. Might be kinda long but hopefully you can find something that works for you!
Day naps
If Jack is sleeping some and Caprece is awake when he sleeps, see if you can take her to bed (maybe even feed her there) with you.
You could put on some kind of movie for Jack, either lay on the couch and watch it together (albiet through closed eyelids

) or put it on and go off to bed - set the alarm clock for in 30 or so minutes or you might keep on sleeping
Time out
It's amazing how great it is after you have a toddler, to sit on the loo and shut the door! LMAO! Even if you just need a few seconds for a breather, put Caprece in the cot (out of Jack's reach), and just take off there for a couple of minutes.
My favourite time out spot is the shower! When I had two, I had them both in the bathroom with me - I'd put the baby in the bouncinette and the toddler would have a couple of toys to play with (and the baby to talk to). It was a fairly short shower but it made me feel somewhat human to have my hair washed etc! Otherwise do a movie (Jack) and sleep (Caprece) while you're in there.
It's a bit cold at the moment but going for a walk with the kids in the stroller (do you have a double?) can really help. The kids are involved in looking all around while you have some space to think "by yourself"... almost.
Make an arrangement with your DP that he has the first 15 minutes of getting home all to himself. Then you have a 15 minute break after that while he sorts out the kids. Have a shower or a change, go for a short walk, have a bubble bath, just sit in the garage for a bit (out of earshot) or lay down. It really helps. Dinner time is late in the day but when you have a new baby, 3pm is about the halfway point of your day!!
Eating
Get into the routine/habit of having 5 meals a day. You'll need it for bf and Jack will need it at 18mos too. Have healthy, nutritious meals and they'll make a lot of difference! Make sure you snack on things like toast, cheese and pineapple, cold meats, yoghurt, drink of milk, cheese on crackers, fresh fruit... they will give you energy for a lot longer than biscuits. (If you like to bake, make up some ANZAC biscuits because they are not too bad - full of oats etc!).
Keep up your water intake, especially when bf. If you find that hard to do, have a drink of water with every meal.
On the tough days
See if Jack can go to a friend's place for an hour or two. Use that time for sleeping and NOT housework!
If you can tell the day is going to be hard work, schedule an outing. It may help yours and Jack's spirits improve to just be out, or with other people. Music/play groups, just the supermarket, to a friend's house... and then tomorrow you can just start again.
Talk to your DP
Having 2 under 2 is hard work. It's important for Ben to know how you're feeling, how you're coping, etc. Hopefully he is already relaxed about the housework, is he able to help in any other way?
My DH is really good with being available to help with pj time after dinner. After a lot of begging

he has made himself available on a usual basis, to help out with the kids - or at least stay in the lounge with the tv off - from dinnertime through to bedtime. That may not be the way things work out for you guys, but it's one idea.
Time with Jack
I found that if I could spend 1/2 hour with Hannah (when she was 19mos) each morning, that her behaviour would be immensely better in the afternoon! Even though I was tired, the housework was calling, Briona was asleep so I had "free time" (for sleep or housework LOL), I needed to spend some time with her then, so that I could see to Briona in the afternoons.
Sitting down activities are great too. This may be harder with Jack than Hannah (how 'boy' is he?) and he probably isn't interested in nail polish! But this is when Hannah learned to do shape sorting/puzzles, learn her colours etc - 19-23mos age.
If it's a fine day, go to the park and just sit with Caprece while Jack burns off energy!
Hope you can find something helpful in all of that.... it's a big job but it gets easier!