She found out her fiance had a drug problem the morning he left the house instead of driving her to the hospital...and then showed up at the hospital later and tried to steal their son from the nursery.
Don't beat yourself up for not noticing the signs.
and who you believe loves you. I'd rather trust and get burned than live my life as a cynic -- and I say this as someone with experience.
Document everything; do not speak ill of her to anyone; and assume anything you say to her, either in writing or by telephone, will be shown to or played to a court.
Hang in there.