The basics in adjusting for drift are: identify it early, adjust quickly, and keep on monitoring closely.
Identify early
It's vitally important in any long cut (not only made with hand tools!) to spot early if you're drifting off your line.
Adjust quickly
This needs explaining. Because most guides have this tip most know you do this by flexing the blade slightly in the direction you want the saw to cut. But the mistake most inexperienced sawyers make — been there, have the T-shirt! — is to over adjust and then inevitably you end up doing this again and again, resulting in a wavy cut.
What you should do instead is flex the blade and then make just a few corrective strokes. If you're monitoring closely enough only two or three strokes can be enough. Thanks to Chris Schwarz I think for this tip.
Other common tips include:
Wax the blade well, and often
Rotate the workpiece more than once
Flip end for end at least once
And for your case:
Don't hold the board at this angle
This is an awkward angle to rip at, even with a Japanese saw where the dynamics can be a bit different. It's much better to have the workpiece upright (vertical) or at an angle (45° probably being ideal) and held in a vice or clamp arrangement that easily allows for release, repositioning and re-clamping, because you'll be doing this a lot! Seriously, resawing a board over about two feet you'll probably want to adjust position at least seven or eight times, maybe more.
And obviously:
Practice
This is obviously a duh but you say it's your first time trying resawing so don't be too hard on yourself, nobody should expect this to go perfectly the first few times.
I wouldn't even want to show a photo of my first resawing attempt (which was much less challenging than this) because the results were pretty dire! Like you I was using a saw not suited to the task, and didn't know all of the tips I suggest here. So the result was pretty horrible..... I got there in the end, but after planing the pieces were a lot thinner than I'd hoped! ^_^
Resawing is in fact so challenging that some experienced woodworkers either avoid doing it as much as possible, they don't do it at all or it was the final (or only!) reason they bought a bandsaw O_O
If you want to do this a lot...
By hand — you've probably already realised this but you could do with having a better saw. Nothing against Japanese-style saws, but topping the list of what I'd recommend would be a European frame saw, a la Tage Frid.
Not manually — invest in a bandsaw, and an aftermarket blade specifically tailored to this sort of cut. Even with a meh bandsaw a good blade can be transformational in how well it cuts (all cuts, but perhaps most any deep rips you want to make).
Wish I'd remembered this earlier, more on ripping in a previous Q&A, Why is resawing by hand so difficult compared to ripping/crosscutting?
