Can I cut the shower seal that is tearing or do I have to replace the whole seal?
I'd try removing the door and see if you can refit the seal into the groove on the door, maybe a little super glue to hold it in place. If it is, in fact, torn or split from th groove in the door, then you'll need to replace the whole seal. If you just cut the loose end off, the door will probably leak at that point.