For you, it's probably mostly in your head. Overcoming strong cravings is tough. Just ask anyone who has given up something for Lent or tried to quit smoking. Meat is a habit. You obviously derive pleasure from continuing to eat it, so quitting isn't going to be easy.
I suggest that you think harder about your motivations for not eating meat. You obviously aren't just doing it for the heck of it, right? Why stop eating meat? Why is that something that you want to do? Write it down, carry it with you, whatever. Just remind yourself why your choice to not do it is more important than the choice to do it.
If you don't have a strong conviction backing you up, it's going to be really hard to say "no thanks" when someone offers you the chicken, pepperoni pizza, or whatever.
Also, for what it's worth, there are some people who aren't suited to vegetarianism. For instance, people with advanced kidney disease (as in on dialysis) often have trouble with it. Ditto for folks with herpes. In both cases, it is because of the balance of amino acids. In order to get enough of one, you often get too much of another, which can exacerbate the respective conditions.
Obviously, someone who is allergic to or intolerant of dairy, legumes, wheat, seeds, nuts, and eggs would have trouble on a vegetarian diet as well.