Mortgage declined after Agreement in Principle — devastated

Had an AIP from Nationwide for £285k. Found our dream house, offered, accepted. Went to full application and got DECLINED.

Apparently my partner's overtime income can't be counted because she hasn't been in her role for 12 months yet (she was promoted 8 months ago, same company for 4 years).

Broker is now trying other lenders but we're terrified of losing the house. The seller has given us 2 weeks. Has anyone recovered from this?