I'm sewing in ends and I have a method to show you that I learned a long time ago which seems to work for me. There is always a gap your cast off edge (and cast on edge too) between the beginning and the end.
It can be closed nicely as you sew in the end.
Begin with the yarn on a needle and held to the back of your work. You can see that my yarn is attached to the Right Side of the gap. Bring the needle through the first stitch on the other side of the gap, from Back to Front.
Now bring the yarn to the back of the work again. Thread the needle through the first side of the gap (in my case the Right side of the gap), from Back to Front. That "Back to Front" part is important.
Now take the yarn and needle to the back of the work again and repeat threading the needle through the stitch on the far side again. Pull the thread snug and see how it looks. I hope it looks great. Mine does.
Thread the end through the wrong side to finish.
By threading the end through from the Back to Front on one side and then from Back to Front on the other side you are making the yarn do a figure 8 with the crossing going into the gap and filling it up. Let me know if you try this. It totally works for me.
Deb