First, take a deep breath. This should be recoverable, one way or another.
It sounds like you trimmed the corners off blocks, thinking you would then sew them onto different blocks. What measurements did you use, when cutting the corners off? Did you cut a full 2.5" in from the corner, or some other measurement? (I'm trying to figure out if the 2.5" squares are still useful to you).
If you cut at 2.5", then I think the only way to rescue this is going to be to trim your blocks. Sew the corners on as you have been doing, then carefully trim it square again. You should end up with a 7" block I think. The pattern will still work, but your quilt will be smaller (you can fix that by adding 1 row/column, if you have enough fabric to make more blocks). You will need to be very careful that you center the blocks when you trim, so that the corner triangles will all line up in the end.
Or, re-cut the 2.5" squares to a larger size, to match your trimmed block (this assumes you have more fabric). You would need to cut your squares 3 1/4", then cut them diagonally in half. The resulting triangle should fit the corner of your blocks correctly.
Another option that occurred to me, but I don't know how well it would work without having the blocks in front of me. Turn the blocks 45 degrees, and sew the corners to the straight (untrimmed) edges of the blocks. That edge might fit the 2.5" square better, and you wouldn't have to trim. But as I said, I can't tell for sure if that will work, and you will end up with bias edges to deal with. This may not be a great option for a new quilter.