We utilise a drill where the kick is broken down into four stages. The stages are:
- Raise the knee (keeping the leg bent)
- Thrust the kick out, and hold it for a couple of seconds
- Return the kick to the position attained at the end of step #1
- Place the foot back on the floor, so you are back in your stance (place the foot, don't just drop it)
This is actually deceptively hard to do because it isn't done fast. It builds strength, and helps train the mind and body in the correct sequence for the kick. This also helps to train you to not kick too high - if you execute step #2 slowly and kick too high you will lose your balance. The four steps are all executed at the same speed - the slower the better.
For snap kicks, steps 2 & 3 get merged together so that the snap out and return are completed in the same movement, the snap is executed quickly but steps 1 & 4 are still executed slowly.
To address your specific questions:
There is always a pretty large 'bend'. This causes me to loose a lot of impact power.
This is most likely due to inflexibility, especially in the hamstring muscle. A bend in your leg at the moment of impact is itself not a large worry, if executed correctly you lose some power but not a lot (and there will be many times that you are not at a perfect distance for your kick so will have that bend).
If flexibility is your problem then that is more concerning - apart from the injury risk (pulled/torn muscles, wrenched joints) your kick could be a lot faster and have more power if you were more flexible. Do bear in mind though that flexibility can be impaired by a number of factors (age, injury, physiology), so train it but don't over do it.
I have problems getting the right height
Define "the right height"?
Usually this is your idea of the right height, and for most people it is chest to head high. You do not need to kick this high, and a kick at that height is not always an effective or quality move. I've been taught that many of the older styles of karate did not teach kicking above the waist, high kicks are more due to the western influence on the art. This is borne out if you view some old footage of instructors/masters training - you will seldom (if ever) see them kicking high. I'm yet to learn a kata that has high kicks in it. Note that this doesn't mean that you don't kick to the head - it simply means that you should bring the opponents head down before kicking it!