I am 27 and feel the exact same way, even when I don't think I am feeling down. My husband just asked me last week. We had a whole bunch of nice patio furniture we were given because people were moving and my husband was excited and while I was happy I didn't feel happy or excited, and I was the one that had really been wanting some furniture for the deck. He asked me if my emotions ever changed and really they don't. I may get upset and just walk away but that's the only emotion I feel.
I also have a hard time talking, and can't keep words straight. It just comes out in a big jumbled mess. Imay have to stop a few times and think about what I was trying to say and restart. But I have also notice similar when I type I use the wrong words like "know" instead of "no" or something like that or just a wrong letter.
I have not found anything to help either, but will listen if you want to talk. It is such a relief to know I am not the only one.