First, congratulations on finishing your first quilt. Binding takes a lot of practice
to get it right. So don't be too hard on yourself. What kind of batting did you use?
If it's polyester it tends to fluff up more. Also, did you square up your
quilt before applying the binding? If the corners are not perfectly square you
won't get nice mitred corners. And my favorite tip when stitching the binding
is to stop just about one short stitch before the 45 degree line. If you go
over...there goes your mitre. Here's an excellent tutorial on binding.
http://pileofabric.com/2013/08/06/ma...ecise-binding/
Practice, make lots of potholders, placemats and study each corner carefully and
you will learn how to get it right. BTW, my mitres are not always perfect but I
think I've improved over time.