I was very timid when I started nursing school
. I was terrified of everything, everyone, and cried all the time. I seriously questioned if I could be a nurse, and it was a very difficult and dark time.
A couple of years later I'm working in the ER and loving every second of it. I tell you that because you may very well be like me, and grow out of it. I'm no longer timid but I'm still highly emotional and sensitive. This timidness that you're talking about may very well thrive and blossom into a confident gentleness that can help heal your patients in some pretty incredible ways. Especially since you'll be working so closely with residents as a nursing assistant. You have the true ability to help people and make a HUGE impact on their lives. You just need time to discover yourself. And its not going to happen overnight.
As far as crying... Don't let your instructor get to your head, I still cry all the time! Recently I sat down in the middle of the ER and cried my eyes out because a patient told me that when I touched her the pain went away. I still cry thinking about it. I cry with families sometimes, I cry when patients die, I cry when I'm happy. Heck, my supervisor teases about how quickly I well up with tears. Now, this isn't about me, but I'm telling you these things because your clinical instructor seems to be trying to tell you that you're incompetent because of it. Im telling you that there isn't anything wrong with you. With time you'll learn to control it better, but you haven't had that opportunity yet. You just need time. There's not only hope for you, but a lot of potential. Like I said, that timidness may blossom into a beautiful gentleness and your patients will love you for it. You still need to do what needs to get done, but this is also the time for introspective learning and self discovery. You seem to have such a great heart, and we need more people like that in healthcare.
The bottom line is this. You worked hard to get to where you're at. Like me, you're probably more timid than normal in the beginning BUT THATS OKAY AT THIS POINT. You're new and need a moment to grow, to discover yourself, to realize that you're stronger than you think. Its normal to be anxious, fearful, emotional, and all that in the beginning. I'm sorry that I don't have any advice on how to deal with this situation, but I wanted to back you up here and cheer you on.