Wow!! That's not good 900 calories a day. You will make your self sick and not be able to lose weight your body will feel like it starving. set your goal on a different date but I know it must be important to you to set that date, but not worth getting sick.Go with at lease 1200 cal. a day.Any sugar free foods are not so great! If you don't know what the ingredients are or can't say it don't eat. Fresh fruits and veggies are great for on the go, good protein and veggies great for lunch and supper, lots of water add a little lime or lemon, I drink lots of tea's plain hot or iced, and water.
Good Luck on your journey, I hope this helped a little.
You need to set yourself a further goal. Eating 900 calories a day is not enough for your body. Maybe give your self an extra month or 2 and see if that brings it up a bit. You will still have lost the majority of your weight by your original target date so you will still be feeling amazing
900 calories is not enough; that's a starvation diet. Not sure what information you put in to get that. Typically, the more you weigh the more calories you will need. It's recommended that females not go below 1200 calories/day.
I'm leery of the "sugar free" foods you're eating; are you eating processed foods with artificial sweeteners? If so, those can actually cause you to gain weight crave sweets.
It's best to eat lots of fresh veggies, steamed or roasted; lean protein, such as chicken, fish, lean cuts of beef, etc. Switch from white bread, to whole grain, from white rice to brown, from regular pasta to whole grain. Get "good" fats from nuts, seeds, avocados, etc.
Be sure to get plenty of exercise. Anything you enjoy and will stick with is good, so the walking and tennis are excellent.