is the grass greener? advice needed

Me and my partner broke up in July, we have still been living together and having sex and things haven't really changed. 
One day we had a massive argument and I went back onto tinder to try and get my mind off things. I came across this guys profile and we really hit it off. I met up with him for a drink and we got on so well and I really fancied him. I met up with him a second time and went round to his, we kissed but nothing else happened. He wants the exact same things as me, marriage, babies etc and I feel so comfortable around him. Every time I get a message from him I feel like a little girl and get butterflies and smile (I can't help it). He doesn't know I'm still living with my ex, he thinks I'm living on my own. He hasn't asked much about my ex and I like him so much I don't want to look like an idiot by randomly coming out with it now. I don't want to look like a liar.
My ex doesn't know I've been talking to this guy or met up with him. But we have still been having sex. I couldn't speak to my ex about marriage or children because I was always worried about his reaction but things are so different with this guy. 
I don't want to keep lying and I know I've got myself into this mess of a situation but I just wanted some advice in how you think I should deal with the situation before even more feelings get involved. My mind is just all over the place at the moment. 😩

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