The head speed while in the main part of the descent should be about the same as you have while flying with engine power.
If your head speed is too low in the descent, then there are two main reasons:
1. Descending too slowly. Fix this by descending faster. You should see or hear the rotor speed up within a second or two. If you don't see the speedup, then you are probably descending at the right speed.
2. Incorrect attitude. The rotor should be almost level with the ground. If you descend as if you are diving like an aeroplane then the air can't flow through the rotor properly. The nose-down attitude you have in fast forward flight is too much for a good auto - you do need to pull the nose up a bit as you start the auto, but not too much, otherwise you will lose all your forward speed. It is OK to dive slightly to pick up forward speed, but you must level out afterward.
You should be in forward flight at a medium speed just before you start the auto. Just before switching to hold, bring the collective to negative and level out. This way the heli is already in auto mode before you cut the engine, so there no chance to loose head speed while establishing the conditions for autorotation.
The other thing is not to flare too early, or too hard. If the heli has the right attitude, you won't be going all that fast. I start to flare at around 5 m (15 ft), but very gently. As the flare starts, I will decrease collective even more, to stop the heli climbing, and to put the heli's kinetic energy into the rotor. The descent rate should stay the same, but you will be losing forward speed. Aim to arrive at the landing spot at a height of 1 m, with zero forward speed. You should have lots of energy in the rotor.
Now try to land as quickly as possible. If you try to hover at 1 m too long, you will lose all your rotor energy, and bounce onto the ground.
If you are getting too low and still have too much forward speed, just slide along the ground, using the collective to keep most of the weight off the skids. The friction will slow the heli down nicely. Sometimes I just slide for the fun of it. With practice, 100 m slides are possible. Of course, sliding is not a good idea on bumpy ground, but it's better then hitting the ground and tipping forward, breaking your blades.
It may help to get someone to watch what's happening, since you have to concentrate on flying. Ask them to watch the rotor speed as you fly through the parts of the auto, and also when you change your descent speed and attitude.
A good way to practice the approach, flare, land sequence is to do it in normal mode. Just to do the whole thing in one single, flowing motion without wasting any time along the way.
The secret to perfect autos is to do lots of them and learn from your mistakes, both big and small.
Man, that's a lot of stuff I wrote
. It's hard to know what your problem is unless I see what you're doing, so I had to cover all the possibilites.