There is no quest; you just missed the trainer. Go to Colossus Blvd in Wizard City. Stand facing Mindy PixieCrown and turn to your left. There is a little road there leading down to a house with yellow windows. Inside is a lady who trains extra spells, including the life trap. Since you are life school, the spell will be free.
Level 46? Wow! I feel really bad that you still haven't got it.
I'm a level 42 ice wizard, so I'm not an expert in Life. But my sister, Rowan Willowleaf, is a Magus Theurgist and she had the same problem. Actually, you can get the Life Trap at quite a low level.
Many people miss the hidden trainer in Colossus Boulevard. Go to the boulevard, and then into the area where Mindy PixieCrown is. Off to the side there should be a house, and though the lights are not on you can go inside. Talk to Mildred Farseer... she should have something for you. I'm pretty sure you only have to be, like, level 10 or something. Hope this helped!