Ah, so I'm not alone! Come, sit with us. We have cake.
It's still a good idea to have the core elements of the ending(s) figured out, at least. That way you can keep yourself on track, knowing where the characters need to end up. Part of why so many AVNs/AVGs flop or get abandoned, or end up with so many issues is down to poor planning (yes, other things too, but that's a big part of it). What helped me as a writer was making a very rough outline of who does what, and when, but also why. You can figure figure out the finer details as you go. When I started doing this, I cranked out a 114,000 word book in three weeks. Just keep the 'outline' vague so things don't get chaotic and stressful when you try to steer things back on course or need to change something.