Any sound you don't have in your own language is difficult because your muscles aren't used to produce it and sometimes because you are not able to discriminate the sound properly. The Spanish rr sound is specially difficult even for native children. It is the last sound children learn to articulate. Some children struggle producing it and need to be helped by a speech therapist. Don't worry about saying rr perfectly, something quite similar is more than ok :)

I am a native speaker, and I was about 12 when I managed to produce the sound. And now, four years later, I still have some problems. There are many native speakers who can't say the "rr" properly, no matter how old they are. Other people have problems with their tongues which makes it even more difficult (that's my case). for example, the small tissue under your tongue that connects the former with your mouth sometimes ends too close to the teeth, which reduces the capable movements.

Tong up towards the palate, the chin goes a bit up also making teeth getting closer but not touching, lips open in order to show the gums but mouth keep small and sound comes from the mouth not from the throat, making the tone vibrating inside your mouth (the top part of it). Basically it's pretty similar to the English "r", the different is that you have to push a bit harder the tongue and you have to be more "meaning" making the sound (could be helpful to imagine you are angry with someone and you are saying to that person just why you are angry). Hope helps,

I'm a native speaker. what is so difficult? in which way I can help you?

Here is a very useful link for you. http://www.studyspanish.com/pronunciation/letter_rr.htm