15 weeks Postpartum diet

(a bit long sorry)So like the title says I'm 15 weeks post with my 2nd little lady and 2 emergency c-section (which has healed nicely). Now I really need to lose the baby weight and mummy tummy, but I also want to make a life change so not technically a diet either. Just healthy eating advice and what exercises you can suggest would be great. I am plus size due to my mental health meds which I've been on since 13 but it makes it hard to lose weight. I'm a bit of a yo-yo dieter I have gone from anorexic to plus size :/. I'm the UK if that makes any difference. I have arthritis even though I am 23 but I was diagnosed as a child with child arthritis but due to my hips being a bit crappy I can't run. Also forgot to add I am not breastfeeding anymore due to milk drying up at 8 weeks. But any advice would be great, like I said any healthy lifestyle tips I'm all ears thanks for reading :) x