is it something that really needed to be a good flute player/teacher? i found it really hard and it is like impossible to do it

As a recorder player, not a flautist...I didn't have to tackle any flutter tonguing until I was relatively advanced, and in recorder repertoire it is only present in relatively modern pieces. Having said that, there's not exactly a huge repertoire for recorder between about 1760 and 1920...though I can't imagine flutter tonguing would have been much used on the flute before the 20th century either. In other words - you could play and teach well without it, but you would be limited a little as to the repertoire you could cover.
Can you flutter without the flute? Doing that pfrrrrrrrr noise horses do sometimes? If you can, I'm sure you would manage down an instrument eventually - it's then just a question of controlling the air flow so that you can make a note at the same time. I don't know how to describe the movement properly - the back of the tongue is held still against the cheek teeth, and you blow fairly fast past a floppy tongue-tip which needs to be curled up a bit.
Just out of interest - is there anyone out there who has difficulty saying an "r" sound but can flutter tongue?

I'm the other way around

I've always done that with my cheeks; is there another way of doing it?

If you roll an r without voicing it, you get something like the same affect

what really make myself think i am a bad flute player was because my friend does't play the flute and he could do flutter tounging

Noooo - the two skills are independent of each other! You can be a good flautist and not do flutter-tonguing, and you can be a good flutter-tonguer (? sounds bizarre....) and still not be able to play the flute at all.
YAP - you're right, it maybe the thing I'm thinking of is when people try to imitate purring. Though a uvular rrrrrrrr is better for the latter.