If it just changed you should see a doctor.
But if its always been that way its a bit of shaping and painting skill. My middle finger has more a fan shape than an oval. I round the corners and skip painting to the very edge. Then when its dry I make sure to keep my fingertips hydrated. I have used olive oil, blistex in complete moisture (hydrates better than chapstick) and gold bond ultimate healing for that) You can also put a clear coat that goes to the edge and clean up the cuticle with a dipped q-tip in remover. This helps blend if your nails appear dry when natural.
Make sure you are pushing your cuticles back all the way and also add some wideness to the base of your nailbed by doing so.When you make that push back look more square than its natural oval it also helps blend in the nail.
Check out some vids on youtube. I thought this one girl had perfect oval tails. It was just the way she laid on the polish. If I find it I will post.
Edit:
http://www.youtube.com/watch?v=wdL_cj8ezDg
Not the one I was look for. But she shows the three stroke method that makes it easy to have nice edges.
Notice how she doesn't take the color all the way to the cuticle edge and it stays on the nail alone.