I'm just starting down this journey myself, and wanted to share the little I do know.
1. IMO, it should be a lifestyle change - not exact "rules" and not a diet. A diet is about things you give up. This attitude can last a short while, but after longer periods it is just no fun. I mean, me never having deep chocolate fudge ice cream ever again (or corn syrup or transfat or put whatever in the blank here...) is just not realistic. However, me cutting down on this ice cream from once a week to once ever month or two, and then eating a much smaller bowl, but really ENJOYING it, well, that is doable and I still feel good. My lifestyle change is less oils, less meats, more vegetables and fruits, exchanging white rice, white bread, white pasta... for more beans, lentils, various other grains.... But I still have white rice sometimes. Your lifestyle change will be something different, that is best for you and your body, but whatever it is, it should be a gentle guideline, not set in stone or blood.
2. Exercise really helps. I will NEVER go to the gym. Many people do and love it, but me being on a treadmill or bike that never actually gets me anywhere is just beyond stupidity for me. But when I am at work all day, then pick up the kids, make dinner, get them to bed, then am on the computer again... well, my body feels like cr*p. So exercise to me is just incorporating small changes. Now that it is warmer, I pick up the kids every day and we bike home. We also bike or walk to the grocery store sometimes, instead of always taking the car. FWIW, the quality of our time is also improved doing this, so it has a double benefit.
3. 5 months pp is really short time. While I was bf my DD, I was CONSTANTLY starving. BF takes up HUGE resources, more than pregnancy! So eat, just eat healthy as you say you are.
FWIW, when I gained the most weight, and felt the worst, was when I had stopped bf, but was still eating as if I was. And getting no exercise. The combo was just BAD in every way. My DD is 3, and starting this february, I started changing what and how I eat, and exercise, and I feel about 25% better. I have more energy, need less sleep, am refreshed in the morning, am not so edgy or sad at work, and have lost about 8 kilos (17lbs). The lost weight is actually the least impressive thing, it is the energy and overall wellness I feel that has had the most impact.
Good luck. You will only feel better!!!