I just aquired for the first time a 29er mtb, the 2017 cube acid! Although it is the right size for me, no19 for my 1.81m height, I feel it's HUUUGE for me, a whole lot of a bike, and I actualy feel afraid ! of it! Will I get used to it and feel safe and comfortable with my new bike? Any method to gain confidence with it?
Not to be trite, but: Get out there and ride!
I just tried out a 29 Plus mountain bike a few months ago, first time on a MTB in years. Felt really weird and disorienting at first, so I just rode on smooth gravel pathways for a while. Once I started feeling a bit more confident in it, I started going out of my way to run over small rocks, twigs, branches, etc., to get a feel for how the bike responds. Quickly built up to "cinder-block" size rocks (breeze-blocks if you're in the UK). It was really fun "relearning" - it's the first time in a long I've felt like a little kid on a bike.
There's no rush, nobody's going to appear out of the bushes to laugh at you when you decide to go around a larger obstacle that you're not yet confident rolling over.
It's not clear from your post what your level of experience is, but if you're feeling really nervous and cautious, just ride on a pavement for a few days. Try rolling off curbs (check for cars first!). Try rolling along on some grass.
Have fun with it!
The answer by SSilk is a good starting point.
Once you have a little confidence, visit a few of your local bike shops ask if they have regular "shop rides" for MTBs. Also ask if they have a "no drop" policy (so you don't get left behind).
It's by riding with other, more experienced riders that you'll see how things are done, how the same situation is handled differently by different riders, and what skills they use. A particular thing to watch is what route they choose in a given situation.
Be sure to ask questions. Of course you'll be the newbie, and get some good natured joking, but persist. You get respect for turning up, and coming back. But also try different groups, you'll learn more.
One last bit of advice: be careful when riding on your own, as you practice what you've been learning. Its easy to follow a more experienced person through difficult terrain, because you're not making all the decisions. But you do have to practice what you've just been shown.
You call that “a huge bike”? I think you have never seen this one. http://bestadviser.net/mountain-bikes/helpful-mongoose-mountain-bike-reviews/ When I was looking for an MTB, I came across this article and saw this Mongoose monster. Later on I saw it in my town, it’s really fabulous and as I realized very comfortable. That’s why it doesn’t matter how it looks, it’s important for you to feel that it is yours, it suits you well. Try other mtb, maybe the problem is not in you but in the bike. Place a saddle lower, it changes a lot. Or choose another type of bike, there are lots of them! I’m sure you’ll find the one you feel safe and comfortable with.