<human>: Can you give me the gist of the text in a nutshell?

Pretty much title, but I'll give some context:

My girlfriend and I are both sophomores in college, going to the same school. I just turned 20 and she is 19, and we have been dating for over a year now. I care about her very much and want to solve a recurring issue that has been happening since approximately the 8 month mark and is affecting our relationship, which means the world to me.

Essentially, while recounting a story about us to a friend, my girlfriend mentioned something I did that was a total dick move approx. 18 months ago and I laughed at the situation, thinking that we were past it and it was able to be joked about. To be honest, most of my laughter was to cover up how embarrassed I was about it and how stupidly I had acted, but she took it as me laughing at her for the same thing again. Her face immediately changed and she got very upset and said, "it wasn't funny then, and it's still not funny now." 

I saw how upset she was, and honestly didn't see the need for such a harsh reaction to something that wasn't meant with any malice or intent to make fun of her, and responded with "jeez, chill; it was over a year ago and I already apologized for it and I thought we were past it." This of course made everything worse, and she now says that me telling her to chill was mean.

When she gets mad at little things like this, it makes me feel unappreciated (I spend 15-30 + hours a week, workload permitting, with her and we sleep together very often) and that I have to walk on eggshells just for her not to get mad at me to have a successful day in terms of growing as a couple. It gets stressful, and as a biochemistry student I can't deal with a lot of outside stress. 

 I'm looking for outside opinions on how to deal with obvious overreactions in a constructive manner that minimizes their occurrence - thank you guys and I really appreciate any feedback.
<bot>: how can I get my girlfriend to stop consistently overreacting in a way that doesn't make her react even more poorly?