"Por tratarse de" (always pronominal) means "since it is related to", "since we are talking about", "being the case that it is", "since we are dealing with". For example: Yo no trabajo los lunes, pero por tratarse de ti, haré el trabajo el próximo lunes. I do not work on Mondays, but since it is you, I will do the job next Monday. Normalmente respondía de manera sarcástica, pero por tratarse de una mujer, fue más amable. He normally answered in a sarcastic way, but since he was dealing with a woman, he was nicer. Iré más rápido por tratarse de documentos importantes. I will go faster since we are talking about important documents. No le importó que se tratara de un niño, lo dejó sólo. It did not matter to him that it was just a kid, he left him alone. Your sentence: No dibujaré mi avión, por tratarse de un dibujo demasiado complicado para mí. Could be translated as: I will not draw my plane, since it is a drawing that is too complex for me. (Note: "tratar de" means "to try to", "él está tratando de terminar hoy" (he is trying to finish today)). I hope this helps.