Hi'ya little redhead!
When I am seaming...I try to resist the temptation to "lace the edges" too far up without "cinching up" or "zipping up"!
Most times, I use a similar color tapestry yarn for seaming, or, DMC perle cotton. Using a different "kind" of yarn helps me SEE the actual seaming "laces". I rarely use the same yarn as the garment. For different reason: the yarn is too thick, or too nubby or fragile. NORO yarn is impossible to seam with.
Congratulations on your efforts to expand your knitting skills by working at your seaming! A well-seamed sweater is so professional looking!
Believe it or not...I didn't learn mattress stitch (had never even heard of it) til about 4 years ago! I took a "Finishing Techniques" class at my LYS. I wish I had learned mattress stitch seaming a lonnnnng time ago! It is so much easier to match patterns and colors or stripes with the right sides facing you!
I recommend clipping the seam together before starting the seam. I use KnitKlips
....Little girls hair clips would work just as well. Take a look at the link...you can see what I mean by the girls hair clips.
I used to use little brass safety pins...but, the clips/Klips are faster to put in and remove and you work the seamline.