I have trouble remembering words and was once advised to use mental pics to prompt me, but it doesn't work for me. A crib note of the first three words of each verse helps greatly, and sometimes just having a crib note available to hand, [but not used], is sufficient for me to complete a song. I do find that when I am learning a song I sing it incessantly, round the house, in the car, or quietly in a supermarket, and I end up singing snatches of it, and eventually I begin to mix up the first half of a verse and tag it on to the second half of another in my head, and the whole thing falls to rat-shit!! Then there are other songs that I have locked into place - mainly ballads - even without choruses - and I find I can reel 'em off without problem even after years of not singing them!!
I advise you to learn a song from cd /'you tube' etc. rather than downloaded lyrics. The songs I learn easiest are those I've had to scribble down as I listen to them. To write it out long-hand I think makes a great big difference. When I stumble over a lyric during a rendition in front of others, I find that saying 'shit' usually gets a laugh!! It doesn't help my confidence much tho'. Sometimes as I sing in public, I start listening to myself singing, and the moment I do and think to myself....'it's going quite well really!'.... and you know what happens then...............!
I have even gone to see a hypnotherapist once to overcome my fears of fouling up, and he concentrated on my confidence, my desire to please and entertain the audience; and more importantly my enjoyment of singing the song for others!!