Great question and welcome to the forum.
It would have been a lot easier to understand if you also had posted a few of the challenges you were having that made you feel unhappy in the current situation.
That said, you've been with this guy since you were 15. Honestly, you've missed a whole lot of experiences and you've got an enormous lot to look forward to. Have you considered having a break? I realise this is going to be difficult, given you're living together however that's going to be the first obstacle you'll need to overcome.
Do some planning on where you can move to, for a start.
Be honest with him too. Tell it like it is. Still see each other BUT as friends. Agree you're going to have to live some life on your own terms so you can better understand where you're wanting to head.
Let me know your further thoughts too – I'm sure I've missed the mark since I've only got a glimpse into what you're looking for.