I have been dating my girlfriend for 14 months now and we always talk about how we want to someday spend the rest of our lives together. We rarely fight and we always make time for each other on the weekends. I love her so much and she is always telling me I'm the perfect boy for her and that she will always be with me.
There is one big problem I have though and that is my sensitivity.
Today, my girlfriend had to make a presentation and I offered to make her a poster. She was grateful but I forgot to do it over the weekend (big exams coming up so it slipped my mind). It was not a poster that was needed at all, just a little bonus to her presentation. Today, she brought it up and she was a little annoyed that I forgot and this is where I went downhill.
Simple things like this simply crush me. I hate annoying her even a little bit and I feel miserable for the rest of the day. Then on the same day, I forgot to tell her that I wouldn't be at lunch. She was slightly annoyed at that, which made me feel 10 times worse. I want to be the best boyfriend I possibly can for her and it crushes me when I fall even just a little bit short.When this happens, I simply shut down. I don't feel like talking to anyone, especially her, because I feel sad and like a failure
We got into a fight about how it frustrates her so much when I shut down and she says my sensitiveness has nothing to do with it. I tried breaking out of my miserable state with her, but she made snappy comments at me which made me feel worse.
I know my sensitiveness makes me shut down and I barely talk to her when this happens, which understandably annoys and frustrates her.
She always tells me how perfect I am and I just want to be the best I can for her, but my sensitivity is not helping me. Even when she says things like "You put a little too much of "blank" into the food", I feel a jab of saddness for failing.
How can I be less sensitive? Am I trying too hard to be perfect for her?

Well..
You shouldn't offer doing something when you know you might not have time for it .
And not talking to her wouldve made it worse.
Like cmon it was a bit your fault for letting her down , it made it seem quite childish but just apologize an avoid fighting with her over little things
And next time make sure you of you suggest something , do it.
Don't stress yourself out over it

Yes, you are trying too hard.
You are young. I assume you are in high school, not college?
You will gain more self confidence as you age. The love of my life was a lot like you at first. If I was the least bit snippy his lip would tremble and I would get more angry at the thought that he might cry. He also clammed up for hours, so we couldn't talk it out.
Yet I loved him for all that sensitivity, so I did my best to curb my typical tendency to be outspoken.
We were both older, so we worked on it over the years, and did change a bit. Not that anyone really changes who they are, but they try for the sake of getting along.
Tell her that you will try if she will too. That's what relationships are all about. Neither one of you is wrong in something like character traits. You are who you are.