I was in a similar spot in the past and a few major things helped.
1. Talk to others in the office about going on "lunch walks" or brownbagging together. I did this and found out that several other people were thinking the same things as me, wanting to save money, but not wanting to miss out on going out to lunches with others. So now we try to brownbag it together and go for walks outside in our lunch time, which is ultimately more enjoyable anyway. Some coworkers may not be interested in this, but the ones that are would be good friends to have.
2. Learn to cook packable food. It's its own little art form, but there's tons of info on delicious packable lunches online. Also, find appropriate containers that seal tightly. I really like my set of Lock 'n Lock containers and this big thing I have for salad. Amazon has tons of options, just a couple good containers makes it a LOT easier to pack lunch.
Also, include a little treat or something you love from time to time. For me, that means fresh fruit and squares of dark chocolate, fancy roasted nuts, or a couple squares of cheese. If it's something you couldn't get at a restaurant, all the better- it'll get you craving your homemade lunch.
3. Even if you aren't thrilled with it, suck it up and eat it anyway. Seriously, just stop making excuses for yourself and eat the freaking lunch. A few less-than-awesome lunches won't kill you, which is something I had to learn. And ultimately, even on days that lunch doesn't feel as exciting as going out would have, when I think of the pile of cash and calories that I'm saving, it's still worth it.
Also pragmatically, ALWAYS pack lunch the night before unless you're really a morning person. When I end up buying nowadays, it's almost always because I got lazy and didn't pack it the night before, and I never seem to find the time in the morning.