Can I get addicted if I keep taking100mg single dose of diphenhydramine every night? (50mg is no longer effective for falling asleep after a few days.

Perhaps. Psychologically, but not physically. 100mg is a high dose, do not climb any higher, and you should likely speak to a dr for an alternative with less side effects.
Can get habituated. I don't recommend someone your age to take chronic otc meds to initiate or maintain sleep. Other causes have to be ruled out. I would start with sleep hygiene. http://www.sleepfoundation.org/article/ask-the-expert/sleep-hygiene.