I struggle on goodbye and goodnight myself, I usually substitute "see ya" "see you later" "s'later" (slang over here) "talk'cha" (”talk to you later”) or even just "later" for goodbye, and "night" or even "see you tomorrow (or Monday, etc)" instead of goodnight. I know avoidance isn't a great tactic but I personally find it helpful in the meantime while I work up confidence with certain words, eg working from "night" to "g'night" to "goodnight"