It's not easy learning new things. Whether it's a new instrument, new song, new solo, new scale, you name it - the first time you try it tends to be a challenge.
BECAUSE IT'S NEW!!!!!!!
We all have neural pathways that get better and stronger the more we practice something. At first, those pathways suck. They're weak! They need to be made stronger and stronger just like building a stronger bench.
It takes time.
It takes some patience.
It takes practice
HOURS OF PRACTICE
STOP getting down on yourself just because you can't play something perfectly, or easily right away. If you have NEVER done something before it's unrealistic to expect yourself to be able to do this new thing right away.
As of this entry, I've been playing almost 20 years. I STILL come across things that kick my brain's ass because certain finger patterns or note choices are very foreign to me. That's fine. That's normal. And, if you plan on progressing as a musician as long as you can, it will happen to you too. OVER AND OVER AND OVER AGAIN!
Now, learning new songs or solos, etc. may actually come very easy to you sometimes. Why? Because you have already played/learned enough similar material to where this new piece of music isn't really all that different in terms of motor skills and muscle memory.
So, if you're one of those people who say things like, "but it shouldn't be this hard!", STOP IT!!!
You are setting unrealistic expectations for yourself. If something is hard then you simply have not performed that task enough times.
Now, here's a video to demonstrate this idea even further :)
John Taylor - guitar and bass instructor for Mile High Shred.