I'm not experienced by any means, but it looks like your tenon shoulders are square. In the pic, the top ones have a gap on the right, and the bottom ones have a gap on the left, which would confirm your leaning to the right. If you can break the glue like woodnthings suggested, you may be able to recut your mortices.
If the glue won't break, I don't know how possible it is, but you might be able to flush cut your stretchers and cut new mortices to the side, where the old holes will still be under the shoulders, but so you're not trying to cut a hole on top of your old hole.
Hopefully that made sense! :)