I can do the skills such as the back walkover perfectly on the floor but when it comes to the high beam i just cant do it no more.
I'm going to tell you something someone told me: the beam will be there. Sounds silly, but that was an epiphany right there and has seen me through many beam crises.
I would recommend that you go line, floor beam, high beam, and give yourself one chance. Do not set up to go until you are ready. Don't stand for three minutes if you have no intention of going. You know when you are standing there and you know inside you're not going to go? That feeling? Yeah, don't do it. Every time you do that you mess with your head a little more, and it's like "score one for the mind block." Get up on the beam, take a deep breath, think of an encouraging phrase to say in your mind that you can keep repeating, then walk up and go. If you aren't ready, don't get up. No "trying." Be ready.
Good luck If it means anything, I know you can do it.