"Worthful" would be a literal etymological translation (but it's not a real word in English). It helps me remember, though.
It could also be "valueful" (which also isn't a word but is closer to the swedish word) since värde can also mean value
And fullt also conveys the idea of ability, ableness (able). While "worth" changes to similar consonants and vowels: w to v, th to d and o to aa.
Given that it is värdefullt, as oppose to värdefull, I'm guessing the noun in question is an ett?
So värde is value, thus värdefullt literally means "full of value", right?