I don't see why you should force yourself to do something you are not comfortable with. On the other hand, I was terrified of IRL support groups, but I made myself go, and I'm glad I did. There were disadvantages, but they were far outweighed by the advantages. And I wasn't all that open, either.
Unless the group is weirdly militant, you don't have to talk (other than introducing yourself). You can go a few times and see how you like it. No long-term commitment.
The other thing is that such groups can be a great way of making one or two friends. If people go to coffee or dessert after the meeting, you might like that better. You could meet someone who has other things in common with you besides being trans.
