Can't Play It

It's really stressful to me when I try to create music. It just doesn't "come" to me like I see it come to others. I want so much to write music to express my emotions. It frustrates me that it doesn't come naturally. Why won't it play like I want it. I can't think up cool guitar parts and play them. Randomly tunes will pop in my head but even if I'm around a guitar I can't make it sound anything like what I hear in my head. God please help me overcome this. Or give me a vocal challenger at least. I can't keep learning by singing along to songs in my car. Something has got to change. God, I'm asking please.