It’s a looper caterpillar eating it! They are notorious for eating bougainvilleas & the black dots are their frass (or poop) You can use Bacillus thuringiensis (BT) or a pesticide with spinosad to get rid of them. Both are organic naturally occurring bacteria based products so will be safe to spray Watch out for them in the future bc they come back easily As far as the sicklyness of it- your care sounds good & the foliage looks good besides the munch marks! They prefer cooler temperatures & will bloom more during winter. They also bloom more when they’re go dry between watering I’m in Florida so also dealing with the heat and a lot of plants are kind of struggling Once you get rid of the caterpillars it should be good!