How did the economy of his thoughts work?

If his mind were a school bus (which he often envisioned it was, barreling, blundering, slow), it would follow that new and old thoughts would need to share the worn bus seats of his brain. However, disparate groups do not often commingle on a school bus. 

The new would not flirt or make polite conversation with the old, but rather would push the old to the front of the bus, eventually causing a few to start asking for rides from their parents to school. 

If his mind were a party (hardly, but there are all kinds…), the new latecomers, still wearing their coats and boots, would replace the old, now marching to their cars in pairs or alone. 

Puffed-almighty with the desire to make life electric, new thoughts would roar through his teetering wax-walled fiesta. Dancing between the warm, affable air of past conversations and the morose wind that allows paint to dry (like an immobile furry corpse, this dead party) they would tease his joists. Between wanting to conk out and be revived again by mischief's breath (a host's purgatory), that was really how he felt. 

Yes, it could be, his mind was something like a party. (Although then who are his guests specifically and what is the occasion for the event, etc.)