"If you did this, then you wouldn't feel so bad..."
"Just try not to think about it."
"It's all in your head, you'll get over it."
"You shouldn't feel like this..."
These are things nobody with a mental illness wants to hear. Life with a mental illness, or even several, is already hard enough. We do not need you telling us what we should or shouldn't be doing, how we should or shouldn't be feeling, etc. Especially if you yourself do not suffer from those mental illnesses.
Many a time I have had friends who — just because they're a psychology major, or have taken Introductory Psych (that's the worst) — feel free to give their unsolicited opinions on my mental health. While they think they're being helpful, as though their advice will somehow be the cure-all for your depleting mental state, it is actually harmful.
Their advice often comes off as condescending, like "I'm mentally fit, and therefore, I know better than you."
This is not the case.
If you don't suffer from any mental illnesses, good for you. I wouldn't wish it on anybody. However, you simply cannot understand how difficult it is to live daily while struggling with your own brain. I would love to exercise more, but the fact that I can get out of bed in the morning is a miracle. Yes, eating healthy does help a little, but if I eat that day at all, it is an accomplishment.
The fact of the matter is: even if you're a psychology major, you are not trained to give that sort of advice. Until you receive your degree and the proper training, please refrain from diagnosing or therapizing people. You are not our therapist, psychologist, or doctor, and therefore, you do not know better than us.
When your friend comes to you to open up about how they're feeling, be a good friend and listen and comfort. If we wanted to talk to a therapist, we would have just gone there in the first place.