Hmm.... You might take a week or two ff and come back to it. For me, sometimes I just need it to sink in. Then all of a sudden, I'll try again and it'd right there.
Something I have forund super helpful:
Get a hymnal. Go to ebay or half.com and get a cheap one. I have one called Hymnal 1982, I think, that's decent. The music is meant to be sung by not only choirs but by non-singer church folk, so there is a great variety of difficulty to choose from. Just use "la" or "ooo" or some syllable, and sit down with a piano (or whatever) and your hymnal and practice trying to sing the melodies. Go one or two notes at a time at first - guess, sing, then play it to check yourself, then sing again correctly, and move on.
Don't worry, it'll come. Keep at it. Try new stuff. Come back to the old stuff.
