TL;DR: You messed up, own that to yourself before you talk to her. Don't lie. Express how your lie was wrong to her, and mean it when you say it. Figure out your needs, and her needs, and communicate that to each other exhaustively, and make a plan around it.

First, realize that you have messed up. Relationships are built on trust, and literally every lie that either of you tells erodes that, however small the lie may be. And this lie is about where you are for almost a third of your life? That's...not a small lie. So if you go into the conversation not accepting, and embracing, that you have done wrong, you're going to come off as unfeeling and dishonest. After you've genuinely accepted that you have messed up, tell her all that you wanted to say here as much with honesty, candor, and directness - and if you believe it it will be apparent in your words. You ask how to do that, but really, honesty and directness is your best strategy here - tell her why what you did was wrong, and how it could lead to mistrust and how it was unfair to her. Identify what you did, identify what was wrong, and say "I shouldn't have done that" or otherwise express regret. But before you start making apologies, be sure that you spell out in detail what the actual situation is - and, again, be blunt.

It sounds like her expectations on you were not reasonable. It's important that you lay out your needs, that you make clear why you felt the need to seek an escape. When addressing a topic like this, it's important not to use phrases like "you pushed me away" or "you made me do this." If you're going to talk about her actions, talk about how you feel and what actions on her part contributed to that, and then move from how you feel to how the way you feel guided your actions. Instead of saying something like "You wouldn't let me pursue my art so I had to find a safe haven," say, "I felt invalidated by some of your criticism, and I tried to set up a place where I could pursue my interests without feeling that way," or something like that.

But that's how to explain away your past actions. Going forward, you guys need a plan that meets all of your needs. You need to lay out what your alone-time needs are, what hobbies are important to you and that you will continue to pursue, and the like - and you need to understand what her attention needs are, and the hobbies that are important to her, and the like. You need to know what each of you is willing to give on, what you can't live without and what you're willing to compromise on. And you need to discuss all of this in explicit and clear language so that there are no misunderstandings.

And it may be that there is no plan that meets all of both of your needs. If your needs truly, genuinely, and fundamentally conflict, the relationship may not be viable, but that's a conclusion that you'd have to come to yourself. I'm sure that this is a bit preachy, but a plan that involves lying is a plan that replaces an explosion today with a time bomb. That's still not a safe and stable foundation.

You didn't ask for solutions to this problem, but I'm gonna offer it anyways - it sounds to me like a great solution would be for you to reserve certain days of the week, maybe even the same days, as "you time" days, with minimal interaction. If you can afford two apartments, you can probably afford one apartment with a couple of extra rooms instead - although 'moving back' may not be necessary, it would probably help with the newly-formed trust issues. In a bigger living space, you could designate some areas as "you space," where you can retreat to if you feel smothered. My girlfriend has such a room in our house, actually.