Burnout is a real risk for aid and development workers, including volunteers.
Observation in the field will tell you that it's extremely rare for a Westerner to spend more than 8 years in a developing country context without becoming bitter. If you want to avoid burning out, spend less than 8 years in the field before you return home (or somewhere like home). Don't even think about moving back for some years after that - even though you may still work in the same area and even pay visits (not too extended).
Your mental health is precious - don't expose yourself unnecessarily to risks of posttraumatic stress disorderW (PTSD).
See "Ten Principles to Help Aid Workers Thrive" from Satori Worldwide and "Isn't it time for a morale boost?" on how-matters.org.