If I were you then yes, I would probably put up an ad. If you find a good and reliable rider that clicks with you and your horse then great and if not you won't have lost anything.
When I was looking for a (preferably) dressage horse to share I really struggled to find one as there were absolutely no dressage horses out there for share. The one I found in the end hadn't even been advertised, so I was really lucky there. This is just to show that there are riders out there waiting for a dressage horse to share.
Just make sure that you draw a good contract, probably with a trial period. You could also make weekly/fortnightly (depending on costs) lessons with your trainer a requirement so that the work you put into your mare doesn't get lost.
Good luck in finding a sharer if you decide to go for it