To keep yourself motivated, set smaller goals at a weekly or monthly basis. Those goals can be small things like trying a new exercise, fitting into a smaller pair of jeans, etc. You'll always feel as if your accomplishing something but you're still working towards your big goal.
I just started Dr Bernstein and want to lose 90lb as well
Dont set a timeline, just stick with it. It's going to be difficult at first but after a while it will be way easier. There don't look for express diets cause they only give temporary results. Just eat healthy and exercise. Actually maybe only 3 times a week isn't enough. I would recommend 2 times strengthening and 3 times cardio. Also you can follow some motivating tumblr blogs they helped me a lot!
I would think a year or so would be a realistic goal. You want to lose about 90 lbs in one year that would be 7.5 lbs a month. That's a healthy rate and you are starting at a great time of year with all the fresh vegetables available during summer months. When you get to cooler weather you can use soups as a great basis to help you loose in fall/winter. Don't get discouraged. Keep at it. Hope this helps.