Hi laureen, and welcome!!
My goal overall is to lose 112 pounds. When I first looked at the numbers, I thought I'd never get there. But here I am, 52 pounds lighter, and 60 more to go.... almost halfway! You can do it... lots of people here find it easier to break it up into smaller chunks and set smaller goals, and reward yourself with a non-food treat when you reach that goal. For me, every multiple of 5 lost (5, 10, 15, etc) was a little celebration for me. When I lost 25, and then 50, I treated myself to a book or two. It's important for me to stay motivated with such a huge goal, but also to recognize how hard I'm working and how much I've accomplished.
How fast you lose the weight will depend on how you put it on, I think. You said you gained over 100 pounds in less than 3 months? If you have any medical conditions that might contribute, it might help to go to your doctor and get some advice from him / her. If it was purely from eating too much of the wrong foods, then it's a little more straightforward (although getting an "all-clear" from your doctor is usually a very good idea when you're starting something like this... I did it, and he was very supportive).
Do you have any plans in your mind about how you'd like to lose it (exercise ideas, food plans, etc)? Time-wise, you'll get a sense of how your body works, but most folks aim for 1-2 pounds a week... it's a safe rate that sets your body to lose fat, not muscle, and it's more likely you'll keep it off if you lose it more slowly than if it drops off. On average, I've been losing 3 pounds each week, but as I get closer to my goal, that will slow down a bit.
This is a great forum, and you'll find lots of great advice here! Just let us know what your plans are, or if you need help making a plan and setting goals, and there's lots of people who would love to help!!
You can do it!!!!!!