OP I so feel for you.
I am a SM to two DSC (age 12 and 10). I have been in their lives for 4 years now and I absolutely adore them, and I think they love me too. We have them with us every other weekend.
My DH and I have a DS 2.5 and we are also expecting our second in May.
DH works full time in a very stressful job with long hours. He earns very good money. I work 3 days a week and also earn good money (I only mention this because my income does also make a positive contribution to the household).
I can say that NEVER in our relationship has my DH taken my contribution to his childrens lives for granted. When I am able then I do help with school pick ups, feeding them etc.
I buy all the DSC clothes for them (partly because I am good at it and enjoy it), and everything else that is needed to be kept at our house (they aren't allowed to bring anything from their mum's house to ours).
I also plan, book and arrange all our family holidays always thinking of what DSC will most enjoy at their age. Again because I am good at this, and enjoy making things special for people I love.
When they are with us they are never treated as anything but 100% part of our family.
BUT - my DH still sees all of their logistics as entirely his problem. He thanks me profusely for every contribution I make, and whenever I say I cannot help, he has never once criticised or attacked me for not doing my bit. He finds another way to sort it out (I admit I am careful not to let any logistics 'problem' transfer to my shoulders - he is far too good at washing his hands if I let him).
But has certainly never said or implied that because I don't work the hours he does that makes me more responsible for issues related to his DC. He was solely responsible for all his time with them for three years before I appeared on the scene, and that has not changed.
My easier hours DO make me more responsible for everything related to our own DS, and preparing for our new baby. I do absolutely everything related to 'running' our son - and I am happy with that, and will do the same for our second child.
I think you are a goddess for everything you have done to date, but frankly things change. You are entering your 3rd trimester as am I (I am also 41 and bloody knackered). I just make it through the day doing my job, dealing with our son, preparing for baby, and managing our household and people who help us.
I have never seen it as my job to get heavily involved with the logistics or development of my DSC - and my DH has never asked or wanted me to either. In fact, as they see it I am the fun, cool one and DH is the grumpy strict one who bangs in about homework too much.
I suspect you have allowed yourself to be somewhat pigeonholed into a role which you are no longer comfortable with (and I entirely see why - your own workload at home is ever increasing). But just because you want to redistribute this load more evenly between you, does NOT mean you shouldn't be able to.
I hope you feel able to be tough on this, as when May arrives I worry how you will cope both mentally and physically.
for you OP.