Feeling defeated

Can anyone confirm or share there story? I was told my b12 was low and now I’m going every 2 weeks for a shot and I’m on my 3 shot and I’m still the same and feel constantly light headed and fatigued. When I asked the nurse she said is takes time to feel a difference. Is this true? I know she knows bette than I do but I just feel like so depressed all the time from this and I just would feel more hope full hear someone else’s story. I just feel like I’m never going to get better