I agree with the "acceptant sadness" of winter, but it is interesting when he implores the reader to "think not of (the songs of Spring)."
Keats is urging his reading audience to stay in the present and enjoy the "Season of mists and mellow fruitfulness" instead of longing for Spring, the desired but unreachable at this time. Moreover, there is the dread of winter that stands between Autumn and Spring, which further complicates a failed attempt to remain in the present. The whole "mellow" vibe and sleepy diction may suggest that the best way to circumvent the melancholy of Winter is to enjoy Autumn to the maximum extent and then hibernate, particularly suggested in verbs like "drowsed", until the melliflousness of Spring arrives and revives.