Thinking hard, going in circles, wracking one's brains, musing deeply... can you help me find new or colorful ways to say this in Spanish?

One of my teachers just suggested "Devanarse los sesos". (Devanar= winding, as in yarn) 

Cavilar = ponder.  Paso mucho tiempo cavilando sobre el ensayo que tengo que escribir.  

Do you know of other interesting ways to say this, beyond simply "pensar"? Are there nuanced ways to distinguish between thinking hard about something meaningful or creative, versus simply getting distracted ruminated about something negative, versus simply trying to remember a fact?

Normalmente hago mis preguntas en español pero en este caso estoy buscando aclaraciones de las personas que hablan los dos idiomas para que podamos entender los matices de este concepto. Muchas gracias!