I think you should go to your doctor. He would probably have no objection to giving you B-12 injections if you're deficient in B-12. If you're not deficient in B-12, injecting yourself is not going to help you lose weight. Better to take an hour's walk in the fresh air.
If your doctor wants you to inject yourself, he'll give you what you need or give you a prescription. Don't order anything over the Internet. You don't know what you'd be getting. There's no secret to weight loss. It's all in eating right and exercising enough and at a high enough intensity. It's a lifelong commitment.
Have a lovely wedding. December weddings are beautiful.
I would ask this to the nutrtionist on the doctor to patient forum.