hi Steph,
I would also wait a bit and test again before throwing in the towel! First of all, yes 12 DPO can be early. A BFN at 12 DPO doesn't necessarily mean you're out. Second, you don't even know that you're 12DPO for sure right? You might be less.
I hope you feel more cheerful soon!
Congrats on sticking to your weight loss plan

Working out is hard to get into but totally worth it. I feel so good after a workout (not necessarily during but definitely after).