External melatonin downregulates your endogenous production. Using it continuously means it's not doing anything special anymore. If you stop it you'll have insomnia for a few days while your endogenous production ramps back up. Really you should be looking at what caused the insomnia in the first place. Coffee too late? Too much screen time at night (easily solvable with sepia filters)? Using daylight bulbs (3000K-5000K) as lighting at night? Literal hospital or insane asylum lighting.
If the insomnia is idiopathic and unsolvable then I suppose you could do some sort of herb tincture rotation. Alcohol tinctures = better because it extracts the components of the plant more. Tons of herbs have sleepy effects. Valerian, passionflower, lemon balm, hops, mugwort (bonus lucid dreams), california poppy, kava, skullcap, etc. I've tried valerian and hops and they worked. Most things stop working if used continuously, so figure out a few that work for you and then rotate them.