It looks great! Mattress stitch is fiddly and can take some practice to learn to do neatly, but I bet you'll see improvement on the second mitt. You can also always undo the seam on the first one and redo it if you're not happy with it
VeryPink Knits is a great resource for video tutorials, including seaming/grafting. She also has slow motion videos of a lot of techniques that are specifically geared toward beginners. She's been my first stop for years when I need to learn a new technique for a pattern.
My best advice overall is to take each new technique as it comes. A lot of patterns seem intimidating on first look, but even the fanciest, most complicated-looking stitches are (almost always) just knits or purls at their core.
There are tons of free tutorials and resources out there, and lots of online and in person fibre arts communities full of lovely people. Ravelry is a fantastic place to start for free and paid patterns (Etsy has a lot of AI slop scams), and I hear their forums are pretty good. If you're a Discord person, feel free to DM me and I can link you to a fibre arts server I joined a few years ago as well (not sure what the rules are here for linking to those sorts of things).