First of all I don't see where it is uneven from the picture. When there is sashing in a quilt it is easy to get things going askew because of stretching. Whenever I do sashing I do it the same as I do borders. Measure the center (in this case the block) and use that measurement and then ease in to fit. I too disagree that you don't ease in quilting making. I have to do some easing all the time with my border and sashing. In the end I have flat square quilts. Sometimes it takes a lot of pinning to get things to fit but in the end I'm happy with the results. After I measure I mark my center spot and then the quarter way spots on both the quilt and the shashing/border and match those spots and then ease in between. Sometimes I will even mark the eighth way spots if the distance I'm covering is long. For me making a quilt with sashing is challenging. Another way to diminish the problem of stretching is to make your sashing with corner squares. If you want a continuous look just make your corner squares the same fabric as the sashing. This way you have less distance to match.
I think you're being too hard on yourself.