I’m not sure what could have made it bitter. With the cilantro and basil, I think it has a sweet type of flavor. You could try a different oil, and be sure you love the flavor of it before adding it to it (oils can have very different flavors and some are not so great on their own, and with a recipe like this the flavor of the oil is very important) If you feel it needs more “sweetness” I’d suggest adding more basil, maybe a bit of lemon juice (which tastes sweeter than lime juice)