Is your uninsulated attic warm enough where you live? In the 30s and 40s F can be too cold. Fluctuating temperatures and fluctuating moisture can also be a problem.
I'd leave the pots outdoors until the nights get into the 40s, then bring them inside, out of direct sunlight. If you have the room, leave the bulb in their pots. Don't put the bulbs or the pots inside anything that will reduce air circulation. The soil has to dry out, or you will be tormented with fungus gnats flying around. Damp soil, with poor air circulation, is a recipe for decay. There is another issue. A lot of commercial bulbs come with a tiny bit of bacterial or fungal rot in the bulb: This can come from wounds during harvesting, the initial forcing, shipment, or insect damage. The first summer, the bulb does OK, but once you bring it back into the house, during the second winter, the rot, that was originally on the bulb, will overpower the bulb. That's why they seem to rot from the inside out, that decay was inside the bulb when you received it. Native members of the Amaryllis family come from the deserts of the Southern Hemisphere. You are trying to mimic a Southern Hemisphere desert winter: decreasing daylight, temperature, and moisture. After 2-3 months you increase sunlight, warmth and moisture to tell the bulb that spring has come. It's an individual preference whether or not to repot into fresh soil. The bulb should put out new growth, and bloom, even if last year's foliage never completely died back. The time of re-bloom is dependent on the timing of the winter dormancy. You don't have to force dormancy. In the winter, amaryllis can be treated like a houseplant: just cut way back on watering to reduce the likelihood of decay: In late winter, the bulb will put up new top growth and eventually bloom. The reason to force dormancy is to get the bulbs to bloom earlier than they would if left on their own schedule.
