One thing I can suggest too is to just be straight with the person (if you aren't already) when you first meet. As in saying "I get very frustrated because I feel like I don't don't know how to act around other people and never know what to say. I really need your help with this, can you do this?".

The job of these support workers is just what's stated in this question. Their job is not to just hang out with you and act like they are your friend. It is certainly not to text while they are with you and also not to coddle you. They are supposed to be a mentor who provides practical social skills training. This is best done out in the community, not sitting in an office though. The point is that you'll eventually become less anxious and able to socialize on your own

If your support person is lousy, do you have the option to go to the agency and request a new one immediately?