Hi and welcome
To be frank, it was the wrong thing to do on her behalf and no, I dont think you are overacting.
However, you have learned a valuable lesson in life, that people are human and even your parents can have insecurities. It seems to your mother this disclosure of yours was a big deal that she could not contain.
To be mature about this, the way forward is to forgive her but not expect in future that you'll expose things to her unless she proves trust is garrateed. This might not occur.
You can have a great relationship with your mother but it will not include trusting her with personal developments and relationships. You are not over reacting yet, try to get this trust issue in perspective...its one component only in your relationship with her. I would discuss it with her though, to point out you love her but her spreading a topic meant only for her ears has done damage and you're disappointed because your confidence has suffered.
If she doesnt acknowledge she's done wrong then you have a bigger problem. If she does realise then give her a chance to explain and work towards repairing your trust.
Being gay I'd imagine is a tough gig not because of you but because many straight people are phobic about it. Its the way it is. Man has a way to go there.
Find friends you can trust to share your personal developments.
We also have a LGBTI thread here you'll find helpful to participate in.
Good luck.
Tony WK
