I have this one shirt with really short sleeves that makes me feel TOTALLY self-conscious about my overly large arms. I feel like I'm a fat girl squeezed into this cute, tiny girl shirt a lot of the time when I wear it (like today at work!)
But the thing is, every time I wear it, I get so many compliments about how nice I look. So clearly wearing something that fits only enhances the progress I've made, not the flaws I still have.
In short: You should wear the new shirt. Even if you feel self-conscious, you will look great, and the next time it will be just a little bit easier to put on something that's not a t-shirt.