I'm not exactly sure how to help. I've been fortunate enough to have never really come across depression, so I doubt I can be much use, but I think everyone should try to help people in whatever way they can....
I too have times when I wonder if I've been wasting the last few years of my life, and I feel old beyond all 22 of them. I know it's different to what you're going through, but I'm sure there's a time either during, or shortly after, university where everyone wonders if they've been wasting their time.
I find it helps me to clarify my motivation. I'm doing this because........<good thing>. if i didn't do it then......... <bad thing>. That way I can be clear in what I want, whether it be from life in general, from a particular area of my life, or even from this week. I can be prone to getting into ruts of doing nothing, not feeling sociable, not doing uni work, not really doing anything. As hard as it is at the time I think it's best just to make yourself do things. Knowing what you want to do, and why you want to do it helps you , gives you the reason to get out if bed in the morning and get on with whatever it is.
I don't know you, but presumabley you're doing something at uni that's leading somewhere, it's the road onto whatever it is you're going to become. The fact that you're still on that road shows that your time hasn't run out, it hasn't started yet.
People validate their lives in many different ways, some people need to be in love to feel happy in themselves, some need money, some do it through work, some do it through sociability. I think the best thing anyone can do is decide what is going to make them happy, and get on with doing that.