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diligo is a verb, so this isn't quite right. Substitute the noun amor.

Also, according to my (fairly exhaustive) dictionary, eternus is not an acceptable form (the correct word is aeternus). However, some on-line dictionaries do list it as a variation. This may well mean that modern users of Latin (i.e., the Vatican) use the former spelling; however, no native speaker of the language is recorded as writing it without the a, so I would strongly prefer to use the latter spelling.

That leaves us with
amor matris est aeternus.

This is a quite acceptable rendering of "a mother's love is eternal." However, there is one noteworthy quirk that I feel obliged to mention. In the Latin, there is no way of telling, without context, exactly who is doing the loving. It says "love of a mother is eternal," and this could mean "the love a mother feels for her children is eternal" or "the love children feel for their mother is eternal." If you want to be more specific, you would have to say something different, for example in aeternum mater partum amat, "a mother loves her offspring eternally."

I think the ambiguity is kinda cool, though.