It really doesn't look too bad at all! A wonderful first quilt! When you quilt it that small amount of fullness will be taken up by the batting. I'm assuming that you are going to also add the dark cornerstones in the next round of sashing.
You really don't want to start trimming your blocks because you will cut off all your lovely points. Looks like you know the length of the yellow sashing strips. I would keep that measurement and cut the rest that length. Add the cornerstones. Start with one side & carefully pin your cornerstones to match the sashing seams in the existing portion. Now you may see that some individual blocks are longer than the sashing. You will need to ease them in. Here's a video on easing:
https://www.youtube.com/watch?v=Eh7_zvHcIio
Basically you find the center of your sashing strip, find the center of the block, then pin this point. You've divided both pieces in half. Now work with one half, finding the centers of the block & sashing of that half match & pin. Continue in this manner until there's almost no excess.
Do this on the entire side, working with one block at a time.
When you sew, you want to sew with a 'baggy bottom' which means that you want the piece with the excess fabric to be on the bottom. The feed dogs will help with any additional fullness.
For future projects, try not to pull too much on the bias. Starch your fabric before you cut. Make all your blocks first and measure. You might find that all your blocks are a bit bigger or smaller than your target size (i.e. what the pattern says they should be). You can then cut your sashing strips to match the majority of your blocks. All the sashing strips should be cut the same length. You will then need to ease either the blocks or sashings on the odd block to make them fit.
Sometimes the pattern is such that you can trim too large blocks down to make them all the same size.