Ana Walshe's Lover Says She and Brian Fought Days Before Her Death

Brian Walshe and his wife Ana fought just days before her death because she missed Christmas Eve and Christmas Day with her family to spend time with her lover ... this according to the lover himself.

William Fastow took the stand in Massachusetts Thursday ... pulling back the curtain on his relationship with Ana ... with whom he says he was regularly intimate in the months leading up to her disappearance.

Fastow says the two even spent Christmas Eve with his friends in Annapolis, Maryland ... after which she stayed with William at his home in the Washington, D.C. area. Ana's flight the next day was canceled due to snow -- but she ultimately decided to drive back to Massachusetts that day.

Fastow says Ana's absence caused an argument with Brian ... though Fastow made it clear it was far from the couple's only issue ... noting they fought over money and Brian's looming federal fraud sentence.

It's possible Brian learned about the affair because of Ana's absence on the holidays ... because prosecutors allege Brian looked up Fastow’s name on his phone on Christmas Day 2022 -- the same day Ana drove back up to Massachusetts.

After Ana's disappearance, Brian called William to ask if he'd seen her or if she was with him ... leaving a shocking voicemail prosecutors played in court Thursday.

As you know ... Ana was last seen alive on January 1, 2023 ... and Brian was arrested on the murder charge a few days later. He was also charged with misleading a police investigation and improper conveyance of a human body -- two lesser charges to which he pled guilty last month. Brian has maintained his innocence regarding the murder charge.