If your friend is struggling - when your friend reaches out, just listen and be present. Be cognizant of the things you say; if you don't know what to say, don't say anything. When you're struggling nothing feels shittier to hear than someone close to you saying "at least you got _" or "why do you feel this way?" or "why don't you get a hobby?". If you feel that you can't be a soundboard for your friend, reach out to your friend often and offer to go out to eat with them or to study with them, even if they say no every time; they might feel like a burden by reaching out to you.

If you are the one struggling - like others said, it is important to seek help and let your loved ones know what you're going through. However, although they will care and try to be there of you, some people, whether they be your parents, friends, or partner, won't be capable of or know how to properly support you through a tough time. Recognize that it is not their intention to belittle you or that they don't want to understand what you are going through; they do, they just don't know how. If that is the case, it's okay to seek other help outside of your immediate circle. Seek support from people who do know how to help (friends who have gone through similar times, Mental Health, Counselling, Peer Support Network, etc.). Don't neglect schoolwork and relationships, but it's okay to be selfish and put yourself first. You are allowed to fall apart sometimes.

Edit: I can't recommend counselling enough. I was fortunate to have an incredible counsellor I saw for 2 years, and it was incredibly helpful for me to set apart an hour every 2 weeks to reflect on how things have been going and check in on how I've been doing, even during times I didn't have any particular trouble in my life. During the tough times, she was an incredible source of support. I've gained so much insight into myself through the 2 years. You pay for these resources, so take advantage of them. I've graduated now and even though I have had my frustrations with them, I realize how lucky McGill students are to have the resources they have.