I was in a similar situation. I fell pregnant with my new partner when DC was 6. I experienced ALL the thoughts that you’ve been having. I too worried about him having to share me with a baby. And how life had got much easier, he is more independent now etc, so why would I change all that? Was I mad? My thoughts also progressed to worrying how I could ever love the baby as much as my eldest, surely that couldn’t be possible?
I’m not sure how far along you are in your pregnancy OP. I worried about all this very early on, my age didn’t help (40). I was terrified something would go wrong. I found having the scan helped, I felt more bonded to baby and in time those worries I had faded away.
Baby is nearly 1 now. DS now 7 is honestly the best big brother I could have ever asked for. He absolutely adores the baby. He wouldn’t change our life back to what it was, and nor would I. It’s exactly as it was meant to be. You’ll see. None of my worries manifested into real life, and you will cope.
And I promise you'll still get time with your eldest, one on one. I take mine out sometimes of an evening to help me run errands (parcel drops) or we'll go out for a hot chocolate. He feels very grown up and I feel, well, just lovely sharing those moments.
Oh just to let you know a little perk that I didn't have with my first DC. I found maternity leave with my first quite lonely, looooong days waiting for partner to get home. This time around, me and baby have company from 3:30pm when we collect DS. I've found that to be a really lovely change having my second.
I would say this is totally normal, and not depression. As PP said this is text book for expecting baby number 2. I certainly wasn’t depressed, before or after. These are genuine worries as you love your DC so much and feel guilt that their life is going to change.
Good luck with your pregnancy OP.