What about small gridwork?
You could do a crossrail, one stride, small upright, two stride, oxer. I find this works as a confidence builder, much the same as free-jumping.
If you happen to have free-jumping facilities then this is probably the best way to allow her to 'figure' herself out over an oxer without worrying about the added weight and unbalance caused by a rider. (Jumping through a chute is different than jumping on the line as she's not going in a circle and it requires a little more athleticism, so builds coordination.)
If you can't make a chute then gridwork with you riding is good too, try to just ride through the grid with little interference aside from keeping her going forwards and straight of course.
Ramped oxers are supposed to be the easiest jump for horses, also solid fences (with boxes under them) are easier for a horse to see and judge distance than airy empty fences. Many refusals at these types of fences are brought on by the rider's own concerns about a fence. So keep that in mind too.
I noticed you said she sees the oxer and takes on a bigger stride before which messes up the distance. When approaching the oxer, hold her on a shorter stride to allow her to get to the very base of the oxer, get as close as you can comfortably take her. Distance/approach to an oxer is key, not speed, speed does not necessarily = spread covering power. Encourage short bouncy steps to the fence, which will present her with the shortest and easiest distance to cover the spread. So hold hold hold wait wait wait and support with your legs, don't let her run at it and if you see a long spot, just circle away and come again.
Hope this helps