Learning one at a time is easiest, of course, but sometimes there are deadlines that require learning a bunch of new music in a short period of time. Just hope you have plenty of time to practice if that's the case.

As Andy7 said, play or sing it over and over and over and over. Muscle memory, including the throat muscles, helps, and deepening the memory grooves in the mind makes the details more readily available for recall.

As r.padgett said, taking a break can help. In fact, it can be essential. The brain keeps short-term memory and long-term memory in different locations. One of the things that happens when we sleep is that the memory in the short-term buffer gets moved to permanent storage. The short-term buffer, like most buffers, gets full and stops adding new material at some point, until it is emptied. Several shorter rehearsals over over a period of days is more effective than one marathon session, especially if you don't follow up the next day.

Imposing a slightly different melody while trying to remember a song is part of the folk process. It's actually a cool thing to do. Enjoy it, and take a bow for creativity. You've made a variant ;^)

John