I agree. I think there's some underlying issues there. Just because your life seems all good on the surface doesn't mean things aren't going on in your mind.
I am in a similar position to you. I don't know if it helps! I relate to a lot of what you wrote in your post. The shyness, being the quiet one. Also thinking about life and wondering about what the point is.
On the outside you'd never guess half this stuff. I'm also a postgrad (although like you doing enough to get by but not feeling motivated etc), have a happy long term relationship in which my partner is really loving and caring etc. But something doesn't feel right. The only difference is I know I have depression and anxiety!
Don't feel like you're bothering the health professionals. It may just take a bit of therapy to sort out how you're feeling

I've been to therapy and honestly it helps!
Thanks for sharing your post!