Hello Amy,
All of us at times suffer from motivation issues, so first off, you aren't alone!
1. Make a promise to check in at dis and blog your daily efforts - exercise, diet, what worked, & what didn't. I think a lot of us have found the WISH board to be a touchstone of motivation.
2. There are going to be days when you are not motivated. Those are the days that you promise yourself you'll walk or hit the gym for 10 minutes. After that 10 minutes, 90% of the time, you'll just go ahead an finish the work out your started. This is PERSEVERANCE.
3. Spend some time determining what changes happened in your life to get you to your current weight. Medications, stress, lack of time for yourself in your schedule, eating out too much? Try to come up with several ways to intervene to get yourself back on track beyond your 3 month weight reduction goal.
4. Stick some photos of yourself or some magazine photos on your fridge on how you want to look. When you open the fridge to just stare mindlessly inside, stare at the photos instead...
As for 20 pounds in 3 months, it really depends on how many calories you can healthfully cut from your diet and how many you are willing to burn off in exercise. Most of us can't maintain a 2 pound weight loss every week. You are going to hit weeks of only 1 or 1.5 pounds - or those ^%$&(%&^^$^ plateaus. Personally, I think 15 in 3 months might be a bit more achievable with less disappointment when you do lose weight.
It took some time to get to your current weight, and unfortunately it takes some time to get it off!

Wish you a magic wand!
C'leen