My buddy's took a long time to flower. Planted it straight into a five-gallon fabric container of 37.5% Fox Farm Ocean Forest / 37.5% FF Happy Frog / 25% perlite (might have been a little more Ocean Forest, as I was low on Happy Frog at the time, and I supplied everything but the light, utilities, and labor). He asked me... IDK, maybe a month and a half in, if I was sure it was an auto. It just kept on growing, lol. Then, oddly, it didn't seem to stretch much (or for very long) during its flowering phase. He only got a bit over two ounces from it.
Had to have been either DDA or SSDC. None of the others were known for turning purple (it wasn't lacking for phosphorous, or enduring cold temperature).
I wonder, had you kept yours on a "long-day" light schedule, if it would have ever flowered/finished. But I would have given up on that hope, too (probably sooner than you, lol).