did you use to play some type of hard contact sport like rugby.i use to and you get big strong muscles in your hips from always pushing off in a sprint like form,but these are no good for martial arts as i found out.what i had to do was firstly a lot of hard stretching to loosen up.but what i found worked well is,a lot of round house or reverse turning kicks and holding a side kick at your highest level for about 30 seconds(this may take a while to get use to)gradully moving up to doing this with resistance such as ankle weights.i find this teaches the hip muscles to loosen and gradully strenthen to these motions.also try doing side kicks and reverse turning kicks in the pool,this gives you a lot of power in the hips so when your in the dojo,you try to kick like you are holding resistance but your not,so it creates a lot more power.also try doing a lot of side and turning kicks up and down the hall attacking pads,you will find the more you do this,the more your hips will loosen up.let me know how you go if you try these exercises.