Hi OP.
I know alot of people are commenting great advice, but i feel i need to give you my story so you know i 100% understand how you're feeling. this exact same thing happened to me late last year. i was 4 months pregannt with our second child, no other issues going on and all of a sudden his behvaiour changed,like a light switch.
he would snap at me and our toddler over nothing, started taking his phone in the shower/under his p[illow when he slept instead of on the bedside table. being short with me,moody and distant. kept saying he wanted to get his own house (we own a house together!) and i was just baffled. on confronting him and asking him outright about another woman, he of course denied it, made me feel insane and stated he was simply depressed and wanted to live on his own.
basically, he was having his cake and eating it. so i did alot of digging and eventually found out he was cheating on me with a 19 year old new girl from work. (we worked at the same place). i packed everything he owned and kicked him out. he didn't fight it, he still tried to deny a lot of it, yet moved in to her house a week after Christmas, exactly 2 weeks after id kicked him out.
take your control back! i wasn't prepared to let him betray me (whilst pregnant) and then have the cheek to tell me he was leaving me, once he was sure that's what he wanted. nope. i packed his stuff in bin bags and he was gone, approximately 6 weeks after he started treating me like absolute shit, and 4 hours after i got my proof of him cheating.
his life is now an utter mess, he has no money and is sponging off a 19 year old (who is just as bad as him). i'm due our baby in 4 weeks and ill stand by its the best decision i made, even if it was horrendously hard and i feel guilty every day for my children.
these are clear signs he has checked out of your marriage, as horrible as that is to read. i'm so sorry you're having to even write this on here. but the one piece of advice i can give is take what control you can back. don't question him, as he'll tell you you're insane. they will deny it until their last breath. get some legal advice, also some financial advice, make sure you will be OK without his wage. get your ducks in a row and try and find as much as you can out. then i'd confront, don't let him bullshit you, or twist it back on you. i also wouldn't give him a choice,just simply ask him to leave. the truth will always come out in the end, if he wants a mortgage on his own, let him crack on and get one. throwing a full family away because they think the grass is greener.
the grass is only greener where you water it.
good luck, you've got this.