I looked at the XP Trike 2 description, but couldn’t find any description of how to adjust things. I presume you got the “support seat” option which includes a wide saddle and the backrest.
I’m not speaking from experience here… But, well, if I were you, I’d see if I could remove the backrest entirely and try to adjust the seat position itself so it feels comfortable. You might also want to replace the big saddle with something narrower. Saddle comfort is a big issue if you’re going to ride a lot, so I think the primary goal should be to get comfortable in the saddle. The XP Trike is not a recumbent, so I would think that it’s still a matter of figuring out how to be comfortable on an upright bike.
Maybe with the backrest put in place after getting comfortable in the saddle, you need to pitch the nose of the saddle up a bit (which, if it’s like most seat posts, you do by adjusting two screws under the saddle — to set the angle). I don’t really know. It does make sense to me, though, that if you’re leaning back against the backrest that it might be pushing you forward. If the backrest is adjustable, try setting it to vertical first and gradually backing it down to find a comfortable spot that doesn’t slide you forward in the saddle.
Again, I think this will require a few adjustments. Good luck!
Oh, hey, either way I wouldn’t rely on the backrest to keep you in your riding position, Cleo. I applaud your decision to be riding in your 80s, BTW.