Things are the best they've been...ever, with us rn and I feel kinda bad saying this stuff, but I just feel like I'm not completely myself because of the parts I "gave" to him. He changes me for the better in a lot of ways, but I'm missing my solitude.
From experience and mostly observation, some alone time in any relationship is healthy and necessary, while couples who spend literally every second of their lives together usually miss out on something. It can be quite a lot of "solitude", it can be much less, that's up to the individuals, but wanting to have some alone time, perhaps asking for it if needed, is not necessarily destructive of the relationship at all, it can be very helpful in fact. And that's even if the relationship is the absolute best thing that's ever happened to you - great for you.
You wouldn't be destroying that, only reinforcing it. Take care of yourself.