It's a massive red flag that you're scared of raising these issues with him. Healthy relationships are relationships where you can talk to each other and discuss these kinds of things.
At the least, you're in an unhealthy relationship. At the most, you're in a financially and perhaps more, abusive relationship-and I'm leaning towards that based on what you have said.
He doesn't love you or respect you. He is quite happy to sit there, stashing away his cash, knowing that you are struggling and he doesn't give a crap enough to help. He knows it's unfair- and he doesn't care. Doesn't that tell you a lot about him?
Of course, if you raise it with him, he'll likely play the bug eyed oblivious card "Oh, I didn't realise, why didn't you say something?" and turn it around on you-but there is no way that he's getting away with paying absolutely nothing towards your shared lives together and unaware of it.
I can imagine he thinks he struck gold with you. There's a woman he can have kids with, who will pay for everything (including his mortgage and children) and do everything, too! So, he can sit on his arse, stashing his cash whilst you struggle for every penny to make ends meet.
The mortgage thing is beyond puzzling, too. So, you're paying the mortgage but it's his house and I'm assuming you're not on the deeds-thus have zero rights over the property should all this go tits up? He's stitching you up good and proper here.
He gets the benefits of having all his expenses paid, his kids paid for and looked after-and not having to pay for his investment as you're doing that for him. So, he gets to keep his money and his asset.
This is financial abuse. Please seek advice from Women's Aid and get out of this as soon as you can. This isn't a matter of having a discussion with him-he's shown you who he is-and what he truly thinks of you-act accordingly.