@WellWTFisthis
How do you 'maintain' until you have your ducks in a row? You do it by using that same hidden strength that allows a human to lift a car off a person in moments of intense emotion. There's really no why or how, you just DO.
I think if you keep your eyes on the prize (getting out and getting divorced in a way that benefits you) you'll be able to pull the wool over his eyes. Keep to your regular 'schedule' as much as possible. If you find yourself 'cracking', don't be afraid to exploit any reason you can think of for being 'off' (headache, cramps, upset tummy) you can get away with, just be judicious. If he has hobbies or activities that take him out of the house or engage him completely encourage him and wave him off happily, assuming it wouldn't be too much of a volte face. If you have a good reason to be out, use it to go for a walk, see a friend, anything. As far as sex, well, you do what you have to do to keep him in ignorance, there's nothing wrong with that. Just be sure that you are using extremely reliable contraception.
As far as telling your parents, give consideration to whether or not they'd be able to know and still act normally if they see him and whether or not they'd be able to keep things to themselves. If not, best to keep quiet, for now. If so, you don't have to go into the gross details. At best you can tell them that 'things aren't good' and you aren't happy and want out. At worst that you suspect him of being unfaithful without going into details.
I suggest before you tell them that you see a solicitor. Find out what's what and what a divorce may mean to you. Take a 'snapshot' of finances; yours, his, and joint incomes and assets. And especially the mortgage details to help you know whether a buyout is practical before you ask your parents. Think also about what you'd like as child arrangements and discuss those too. Knowledge is power, forewarned is forearmed. You don't have to 'do anything' you are just educating yourself.
If you have one friend or relative that you trust implicitly to keep your confidence, tell them now. Having someone to lean on and blow off steam to can help immensely.
Remember, keeping the status quo will benefit you greatly in the long run. Blowing your top now will not. You'll get your moment to give it to him with both barrels. Til then just keep repeating "The time will come, the time will come". Because it will.