It seemed, and therefore for our losses sad;
I meant to send this heart instead of mine
But oh! no man could hold it, for twas thine.
Apparently the heart was sad for our losses because it was no better than might be made by art. The confusion arises from deserting the punctuation of 1633. 'For our losses sad' is an adjectival qualification of 'I'. 'I, sad to have lost my heart, which by legacy was yours, resolved as a pis aller to send this, which seemed as good as could be made by art. But to send it was impossible, for no man could hold it. It was thine.'
Huyghens translates:
Soo meenden ick 't verlies dat ick vergelden most
Te boeten met dit Hert, en doen 't u toebehooren:
Maer, oh, 't en kost niet zijn, 't was uw al lang te voren.
But this does not appear to be quite accurate. Huyghens appears to think that Donne could not give his heart to the lady, because it was hers already. What he really says is, that no one could keep this heart of hers, which had taken the place of his own in his bosom, because, being hers, it was too volatile.