I really suggest keeping her. Like said before me, it is not fair if you can afford to keep her. While she’s not able to ride, you can still get to ride other horses, and rider her next year. Owning a horse is not all about riding, so she is still good for many other things 😁
I really hope this helps, - Zoe
If you can afford to, yes. It wouldn't be fair offloading your lame horse onto someone else. If you can afford to further, a good physio etc might be able to speed up the healing process, depending on why the mare is lame.