I haven't done Insanity, but I have done P90x. The idea is the same. The workouts are crazy hard.
Jillians 30 day shred workouts are great workouts too (certainly not easy) but I would say that they are not as hard as P90x.
What you need to do is figure out a plan with mini goals. Write it all out. What do you want to accomplish in 1 week? Whether that's working out 4 times, or losing 1 pound, whatever --> write it down. Stick to it. What do you want to accomplish after 60 days? Lose x number of pounds? Fit into a pair of jeans? Whatever, write it down and keep the ultimate goal in mind.
Remember, no matter what workout you choose to do, your fitness and ability to lose weight is at least 70% due to what you eat. I could work out for 5 hours every day, but if I had a Big Mac from McDonalds for each meal, I'm not going to be very fit or lose much weight. Both insanity and p90x has eating plans, as does Jillian MIchaels. Again, pick something that you are going to be able to stick with. Eat small meals several times a day. And lots, lots water. And keep your salt intake low.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.