You need to remember the context of this speech by Demetrius - the Athenian lovers, having been "sorted out" by Puck into their proper pairs, now wake up and are found in the wood by a hunting party including Theseus and Hippolyta. The lovers themselves are trying to make sense of their new reality - that Demetrius, having detested Helena for so long, now has abandoned his love for Hermia, and has realised that Helena is the true love of his life. Of course, he is trying to make sense of the change of his affections, just as Theseus and Hippolyta struggle to understand it.
Demetrius thus commences by saying that when he heard that Lysander and Hermia were going to elope, he decided to follow them in anger, with Helena following him out of her love. However, now, somehow, Demetrius is not sure how, but his love for Hermia has "melted as the snow" and he now views it as if it were a favourite childhood toy that he used to play with lots but now is forgotten. His true love now, the only object of his affections, is Helena. He was going to marry her before he saw Hermia, but then explains his infatuation with Hermia as if he were sick - he didn't want to eat what he knew was good for him because of his illness. Now, however, he says that he is back in "health" and realises what is best for him - his relationship with Helena. He ends his speech with a declaration that this relationship with Helena is what he wishes, wants and longs for, and he swears eternal love.