Am I depressed or just stressed?

First off, CONGRATULATIONS!!!! That's exciting news to hear that you are getting married. With all those big changes and current planning, no wonder why you are feeling exhausted and emotional. It is a lot of work preparing for such a big life event. 

That being said, its possible to be both but I can't really diagnose or assess based on a question. You asked more specifically, how do you know the difference between being depressed and being overly stressed. That is a great question and I would take a look at what are the things that are causing you to feel stressed and if you can eliminate that stimulus and still feel down it may be something other than just being overly stressed. 

There is some research that was published by UCLA on the Four Rituals of Happiness that may help you out given your current circumstances. The first ritual is the act of finding something you have gratititude about. What's great is that you don't even have to find something, it is simply the act of searching that contributes to happiness. The second ritual is naming and labeling the emotion. Our brains and bodies like to label things and it helps to process our situations. If we fail to name them we tend to stuff our thoughts and feelings into an emotional junk drawer, one that is full of stuff we never use but when we open it to try to cram more stuff in it adds more stress and anxiety to our lives. Dr. Dan Siegel stated "Name it to tame it" and I would reaffirm that research has shown time and time again that naming and labeling our emotions helps us to process through them and feel happier. The third ritual of happiness is the act of making a decision to better our circumstances. The decision doesn't have to be perfect, but rather just good enough to help us feel like we are doing something. Much like you posting on better help about the difference between stress and depression, thats one step to finding answers. I would challenge you to look at what can you do to help better your circumstance? What can you do to help things feel less stressful right now? And it doesn't mean you need to "fix it" but feel like you have some semblance of control. The fourth and final ritual is Touch. Our bodies release the hormone oxytocin when we connect and touch others. Oxytocin is the love hormone and helps contribue to feeling happy. When you or your fiance are feeling stressed, overwhelmed, or just at any point in the day... give each other a long, embracing hug and allow yourself to just be connected with them in that moment. 

I don't know if that really answers your question, but I am truly excited to hear the great news and the big life changes that are ahead.