okay, i totally know what your going through! here are some steps to help (i saw a sports psychologist about this lol)
1. identify your problem. Ask yourself why you are getting so nervous?? write them down so you don't get confused or mixed up. make sure its a clear reason too, don't just right i don't know. but really think about it. what gets you nervous? why are you scared? what changes when you go from a line on the floor to a high beam?
2.come up with some solutions to every fear you have no matter how small. if you are afraid of hurting yourself, then ask yourself "will i honestly hurt myself on this skill??" and the answer probably will be no. put mats under the beam to make the beam appear lower and then take out mat by mat. but make sure you stay on the same beam when your making the progress so if you take down a mat and then don't go for it then you can say to yourself "I just did this same skill with an extra mat under the beam, but i did it on the same beam!" do you understand what im saying?
3. don't think about it so much! your just like me in that you get frustrated. but that could also be your problem. i get SOOO FRUSTRATED when i fall on routines, even if its just in practice, and especially on beam. sometimes i cry sometimes i just say WHYYYYYY. but i'm telling you, DO NOT GET FRUSTRATED WITH YOURSELF. getting frustrated does not help. it only makes your problems worse. if you get frustrated after you fall then your more likely to fall again. so if you do fall, say oh well i'll try to go straighter, higher, faster, ect. next time.
4. think about something else either than beam. for example i sing a song in my head while im doing my beam routine. it helps i swear. and right before i go to do a skill, i will say a few key words to myself.
5. THINK POSITIVELY!!!!! okay if you go for a routine and the whole time your thinking i hope i don't fall, or you better not fall this time, or if i fall im going to be so mad, then you will most likely fall. okay heres an example. if i say don't think of pink elephants right now!!
what did you think of??? pink elephants right? thats because the only thing your brain processed was pink elephants. your brain doesn' t pick up on things like the word don't. so if you tell yourself you better not fall right now! then your brain is probably going to pick up the word fall and then you probably will fall. so before a routine say to yourself. i can do this. i am going to go straight and i am going to go for it.
okay and lastly just believe in yourself. if you believe you can do it then then you can and if you believe you can't do it then you can't. you can do what ever you put your mind to and if you go into beam saying oh im just going to fall again, then you probably will fall. so just believe that you can do it
hope this helped!