1- First I would just ask. They may just let you out of the listing agreement. I've done this before. Most Realtors don't want a listing if the seller does not want them!
OR
2-Try talking to the agent and agreeing to pay a commission if anyone she has shown the house to becomes a buyer within the next 30 days or the balance of listing time ( second choice ). Hopefully it will be a short list. Agree on the list, get a new Realtor, and be sure the list is incorporated into the new listing agreement. That way you pay commission only one time. If it sells you and at least one Realtor will be happy. You may want to talk to your new agent first. If they agree, then approach the old Realtor.
3-Always sign for a short period if possible... 90-120days ....that gives them time to sell and you time to decide if you like their service.