First, you are not pathetic. A lot of us are introverts, and that's why we're able to be here for each other through this site (among getting knowledge of the wonderful world of betta keeping). Your husband yelling at you when you try to talk to him is extremely detrimental to the whole situation. Though I know how you feel, seeing as talking to anyone in my immediate family typically ends with the same response. You're right. Your sister in law is being ridiculous. It's understandable for her to be upset with HER husband, but to be upset at you guys is just ludicrous. The whole manager thing...well...she's just blowing smoke. Someone, ANYONE, who treats another person like that is surely lacking something in their life. Typically people will bash on others, because they're truly insecure themselves. By bashing on you, she is attempting to fill a gap in her life. She'll learn that this won't help eventually (hopefully), but until then she'll probably continue. I'm sure you can get a job. Just try finding a job somewhere else; anywhere else. Don't worry about the stutter. Just laugh it off and say you just can't talk today. I stutter when I'm feeling insecure as well.
If you're feeling especially depressed, psychology has shown that people who are in a depressed state typically cannot function normally. To the extent that their brain neurons literally do not fire at the rate of a "happy" person's. This would explain the stuttering. Nothing feels worse than having a thought and not being able to put it into words.
I really hope that things get better for you. Even more so, I hope that your husband can/will start listening more than talking/yelling. Maybe next time you can tell him that you just want someone to listen to, and not someone to give advice (BEFORE you start talking about what's getting you down).
Rant: When someone is complaining about you not doing something, but does it before you can and then continues to complain while they're doing it, and then complains more.
If we are made of cells, then a scientist who studies cells is actually a group of cells studying themcells!