I am assuming you researched juiced fasting prior to taking the plunge as this would give you the answers to your questions as to why people juice fast in addition to what to expect on a juice fast.
There are many entries in my blog that explain the reasons why juice fasting is a terrific way to rejuvenate the body, lose weight and retrain yourself to new dietary and eating habits. I've listed some of them below for your convenience.
http://curezone.com/blogs/m.asp?f=573&i=5 Benefits of Juice Fasting
http://curezone.com/blogs/m.asp?f=573&i=31 Ups and Downs of Juice Fasting
http://curezone.com/blogs/m.asp?f=573&i=18 The Healing Power of Juice Fasting
http://curezone.com/blogs/m.asp?f=573&i=44 Eight Benefits to Juice Fasting
http://curezone.com/blogs/m.asp?f=573&i=48 Off the Top of My Head Notes for New Juice Fasters
I think you know why people juice fast, in addition to why you embarked on this journey. But because you're feeling so badly today, your mind can't get into gear. That's normal. It depends on how badly out of shape and unhealthy your body is that will determine the severity of your detoxes. But trust me, the longer you juice fast, the less severe those detox moments will be. The longer you fast, the stronger you will become. Read my blog in its entirety. Start at the introduction and just take some time to read. The blog will give you tons of information, explanations, tips and tricks and encouragement. And when you're done with that, read Julie's She hated the veggie juices, could barely stomach them. She had nasty detox moments and she got through it all. It was really rough for her. Read her blog and the encouraging messages from others to her. It may give you some insight as to how she got through it. We're all here for you. We'll encourage you, support you, cheer you on, but we cannot make you continue. That has to come from you.
Julie's Blog: http://curezone.com/blogs/f.asp?f=566&t=36527
Detoxing will effect you both physically and emotionally. Your emotions will fluctuate up and down and up and down and even sideways. Today you are undergoing another nasty fugly detox moment. When you have those detox moments, it is imperative that you drink MORE juices (even if you don't want to) especially the GREENS as they have a unique way of coating your nerves making you feel better emotionally, in addition, they work against the hunger feeling and give you more energy. If you are actually feeling PHYSICAL hunger (as opposed to psychological hunger) then you need to listen to your body and drink more juices. Drink more water, lots more, it flushes out the toxins. Drink come herbal tea such as Chamomile as it will calm you, in addition to flushing out the toxins. Enemas also help the detox moment to move much faster. It's when you don't feel like doing any of these things, that you must find the wherewithal to do them anyway. They will make the detox moment move on much quicker. And you will feel better much faster.
Juicing detoxes the body which is the best gift you can give yourself even when you're feeling as badly as you are. Those toxins left alone, will one day become cancer or another debilitating dis-ease. (Dis-ease equals DISSING YOUR EASE!) Allowing the toxins to build up will age you before your time. They will make you feel blah and fatigued day in and day out. You'll begin getting those mysterious aches and pains in your joints and muscles if you haven't already. Your patience and tolerance levels to stress will shrink down to nothing. One day you will become very ill and feel so much worse than you do now.
Perhaps you could shrink the 40 days down to, say 21 days. Get to Day 21 and see how you feel... hopefully by then, you will begin seeing the benefits and feeling the highs and will be strong enough to continue to 30 days. When you get to 30 days, see how you feel, and perhaps then you can get to Day 40. Take it in increments. Cut it down to bite size pieces. Give yourself persmission to do this. I'd much rather see you do this than to quit altogether. Imagine how badly you will feel toward yourself if you quit. And if you do decide to break the fast, please try to do that on a high, rather than a low. I explain this in the Notes to New Fasters (link above).
By removing the toxins, you're giving yourself a whole new lease on life. You're setting an example to your family and friends by taking responsibility for your own wellbeing. This is not to mention the enormous pride and self-respect you'll gain for yourself.
I hope this has helped you.
I wish you the best!!