use whitening strips.

Try taking vitamin supplements. My teeth were pretty bad for a few years because I had an eating disorder and I had all kinds of deficiencies, but they've really brightened up again now that I'm eating properly again. Maybe it's a deficiency that's causing your problem, too?

i have a similar problem, but try to brush 3-4 times a day, that really helps (also longer than 3 minutes each obviously)

My teeth aren't stained but I use a tooth whitening toothpaste. It's kept them white since I started using it. I'm not sure if it helps remove stains, I think it only helps in keeping your teeth white. No harm in trying it though. I've read a few things about making your own toothpaste with baking soda. Again, not sure if it will work, but no harm in trying.

i hear the baking soda trick is bad for your teeth in long terms

Yes it is as it's an abrasive. I'd suggest using it once or twice to reduce the staining and then use a tooth whitening toothpaste to maintain it.