My avocado tree is about a year old, in a pot in the house, about 2 ft tall. In the past 2 weeks it has lost nearly all its leaves. It has new growth forming at the top, but even the newest full size leaves are drying and shriveling at the tips and probably won't last much longer. I water it weekly, keeping the soil moist. It was doing really well for months and I feel it has suddenly taken a turn for the worse. What's going on? What should I do?