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Thats the same problem with me. I guess you just have to train and teach yourself to kick and punch at different levels of power. 3nodding

Getting a bit ferocious is not something you want to lose completely. Being able to land attacks with control is something that is important to develop in the more traditional martial arts but self defencisively instinctively going full force is a far better habit to get into than pulling your blows. I've heard numerous accounts of blackbelts too used to semi contact sparring getting involved in real situations being beaten to a pulp because they instinctively held back.
Having said that being in control is a very important characteristic to develop. There being two types. There's physically landing your blows softly, which kai kazuku has already given some good tips for. Another method that worked for me was by practising hitting on a surface that hurts if you strike too hard, like a wall (using no padding). If you can deliver lots of controlled fast strikes to a wall without raising any bruises, it helps a lot in not taking your opponents head off.
The other is mental control, the more important of the two in my opinion. Physical control is merely a matter of mastering the machanics. Mental control is harder to develop and takes longer, and is a state you want to be in any situation, not just a semi contact sparring situation.
Its about understanding and using your adrenilin response. When your involved in anything... exciting I guess would be the word.. varying dosages of adrenilin will be pumped into your system. From a bit of a nerve tingle to a full on head busting sweat explosion. It has numerous effects. Physically, your heart beats faster, your breathing can become uncontrolled, you sweat, you shake, you feel sick, your face goes pale, feel the need to urinate, become stronger and almost immune to pain. Mentally the first effect is the fight reflex. You become angry and aggresive and seek to lash out at any threat, this is possibly the reflex you're encountering. The second effect is the flight reflex, you become afraid and seek to escape from any threat. After a heavy adrenilin dose, shock sets in.
Understanding this helps a lot in the control of it. When you start feeling the effects of adrenilin, start taking deep measured breaths from your belly. Try to relax your body and mentally step back from the situation at hand. Personally I find smiling, laughing and approaching the situation with a sense of fun works for me.. can be bloody unnerving for opponents tho.. grinning like a maniac whilst smacking them in the face..
Being in mental control in a fight is a strange duality. A) you want to be thinking about what you are doing and fighting smart, and B) reacting instinctively without too much conscious thought.
Understanding your adrenilin response and controlling your emotions helps with A), allowing you to follow a general strategy, i.e. this person is big with fast hands therefore I want to stay at long range, or this person is quick and leggy therefore I want to be close to them. etc. And constant practise and drilling the basics over and over helps with B), allowing your techniques to flow without too much higher brain direction.

Control and power go hand in hand each one being enhanced by the other
Just the other day I got kicked in the chest by a black belt in free sparring practice and I must say that this guy is notorious for having a lack of control so now I have a nice mark on the right side of my chest and the memory of having the wind knocked right out of me
The sad thing is that even after a few days I still can’t breath quite right when I go in and train and my left punch is sorely lacking due to the pain
the biggest problem is that a guy with enough power to break three slabs of marble (10 lbs a piece) at once with a straight punch needs to be able to control himself I mean I work on control every day it is part of the martial arts and I for one think that everyone should treat it as such

I had the priviledge of watching two black belts in Montreal practicing during a recent trip. They were sparring bare knuckled, the power of there technique was unquestionable. The skill and precision in which it was delivered was awe inspiring. After seeing it, I'm currently refocusing my training. These men could have destroyed each other, but they were smiling and laughing the whole time.