It takes some people (like me) a long time to work up to 10 days or longer. Fasting even one day a week is better than doing nothing at all. I felt awful the first five days, after that the second part of the fast was much easier. By day 10, I had a great sense of well being and peace. Next time I am going to set my goal for 21 days.
If your body is very toxic, you will need to start slowly flushing out these toxins. Something called a "cleansing crisis" or "healing crisis" happens when you have a lot of accumilated toxins dumped in your system. It feels like living death and is why most of us break our fasts at the beginning. The quick fix for one of these events is to retoxify ourselves. Its almost like a drug or alcohol fix. A series of short cleanses or fasts will help you avoid most of the really severe cleansing crisises, although you will have some every time you reach a deeper level of cleansing.
Good luck on your efforts. Don't beat yourself up if you fall short of your goals. Just try to eat a healthier diet every day, and keep on trying. You'll make it. You will also get a lot of support and encouragement here.