Being alone is sooo much better than being in a relationship with a cheating lying arsehole.
This. I'm almost excited for you to get through this shit bit to the other side where you think thank FUCK I found out!
Sort out all your logistics now. Flat, moving, telling people for support, any joint money, valuations, etc. Get a plan for yourself in place with one or two people who can help, advise and support you - along with the MN army obviously.
Having a plan will give you the strength to end it because it won't feel like you're going from what's familiar to a gaping hole of uncertainty. You'll know where you are going to an extent because you'll have a plan and some handholds.
Then when you're ready, calmly tell him it isn't working and you want to split up.
You can give him a reason - you know he is on tinder, won't be disrespected in that way and it's made you 100% sure you want to split and won't be changing your mind. Or, you can simply say the relationship isn't working for you anymore, it's over and rinse and repeat until he understands.
I'm sorry OP but in the end, finding this out now will be one of the best things that ever happened to you 