You could try prune juice - its a great natural fibre juice and it may not taste that great but it works very well for me and even my kids.
I also had this problem early on, and I feel your pain!!! It was terrible.
For the initial issue, I took Metamucil (straight-up fiber) pills - the maximum they say you can take per day for something like 2.5 days straight (I think that was 15 pills each day!!!). Then, when it didn't work after 1.5 days, I took Milk of Magnesia (I think that's what it was called) - whatever dose was recommended to cure constipation once. And THEN, I drank some strong coffee with my fiber pills in the morning and made sure I was near a bathroom for the next few hours. The whole time, I was also eating prunes and high-fiber cereal. I was about 9.5 weeks at the time.
I guess this probably sounds a bit extreme, but it worked. Since then, I've kept up the fiber pills (but I'm down to 2-3 per day instead of 15), a bit of dried fruit, and the high-fiber cereal. I haven't had another problem and I'm 14 weeks today (knock on wood!).
GOOD LUCK!!!
I had constipation early on. I started eating a high fiber cereal every morning. granted it's not the best tasting but it worked