But the problem i have now is that when i play between black keys i need to press the keys harder. And thats going not very well. Its hard to play smoothly. Any advise/suggestion for that?
A couple of things.
First, when you say you "play between the black keys", if you are meaning this literally, i.e. the narrowest part of the white keys between the black keys, then I suggest you adjust your position for the optimum placement on the keyboard.
If you imagine a straight line on the white keys where the black keys start, this is your "neutral" placement as opposed to way out on the edge of the white keys, or way in between the black keys toward the fall board (wooden keyboard cover).
With regard to more pressure being needed (and I think if you move to the position I describe, this should help with that) the hands/fingers are really designed to do one primary thing and that is to grip.
The muscles/tendons are already very strong, and as you know you're likely able to hang your entire body weight by holding on to a tree limb and dangling there -- or do chin ups, or other such things.
However, if you are pushing your fingers outward, i.e. away from the palm, or they are slipping away because they are not "gripping the key with the finger tip/pad" and pulling the key down and slightly toward you, then you are not using them to their advantage and they will feel weak.
Make sense?
If the piano was floating on air, the correct playing of it would constantly mean you are pulling it toward yourself.
So its important for your hand to be "alert" and formed to the "shape" of the "key cluster" you're going to play, and that each finger pull the key down with the feeling of gently gripping the key and pulling it toward you. (not a rigid hard hand!)
That's quite a bit of information, and if you can work on that with your Hanon in D major, I think you'll see what I mean.
Imagine the shape of your hand "holding" the D F# G A B as if you are about to "grasp" it gently, then play each key in the group as I described and see if you don't have a more "sure footed" feeling.