Welcome to RYL, I'm glad you found us and I hope it's helpful for you being here. I'm sorry you're feeling low and not treated as the person you are. I don't think it's possible to keep yourself happy unfortunately, we all get low, especially when you have depression. But of course there are things you could do to lift your mood. Are there things you enjoy? People who make you feel good? Uplifting videos? What makes you feel low? Maybe you could try tackling the things that lower your mood. How are you getting on with your therapist?
I can understand how you feel when people treat you differently. Are you at school just now? What people do you have in your life? I guess just being honest and saying that you are still the same person they knew before they were aware you have mental health problems. Simply asking people to treat you like a human and allow you to get on with things could be useful. You can't really change how other people perceive you and treat you as such but you can challenge it. What you think about yourself is the most important thing, so make sure you are treating yourself with kindness and respect.
I hope you can find ways to make things feel a bit better for you.