Hey, the fact that you used the word "insecurities" is pretty telling in itself. You recognize, on some level, that what you perceive as physical attributes that may need work are mostly you not being fully satisfied with the way you appear and not an inherent physical defect.
Here's a tough concept to get ahold, but getting ahold of it is vital: So what if she laughs?
I know it sounds way too simplistic to be important but think about it. You are the exact same person before and after the event, compared to your entire lifespan that's just a split second and she has taken nothing of real value away from you.
And tells much more about her character than your physical make-up.
People can only take advantage of you with your permission, so don't give some random bimbo so much control over your own life.
Now: That being said, you're in the 100% perfect place to make the gains you want to make. There are literally millions of resources on the web concerning fat loss and regaining health and thousands of posts right here to help yer cock size.
Get diggin', get readin', then get doin'.
This might not be the quick all inclusive answer that you may have wanted, but anything of value requires effort and sacrifice to one degree or another.
Good luck !