Moving the seat up will likely help, but don't beat yourself up if you still struggle. I also felt like my spatial awareness was all over the place at first. I kept scraping against the kerb on the left side (especially if there were cars coming on the right). Backing up terrified me because I just couldn't keep the car straight. It made no sense to me why it would feel so different, but it did. I can now drive comfortably on either side, but it just takes time to figure it out. I've driven with enough American drivers to notice that so many of them swerve too far over like I did while it was new. Learning to drive the manual at the same time would feel overwhelming. I'm glad I had that down already at least. Sorry you have to do both.
I think a lot of Brits think they understand because they've driven on the right in the continent, but I maintain it just isn't the same. They are accustomed already to the narrow roads, to people driving over the median lines while they are coming at you, to parking facing either direction, etc, and they grow up travelling and seeing roads that are different to ours. There is so much that just feels different as an American besides changing sides of the road, and that in and of itself brings a whole change of perspective that made me feel awfully silly when I struggled to do things that were so simple before, like backing up (not to mention things like parallel parking).
You'll get there, it takes practice and time, but it will come, just stick to it!