Put the baby higher. Make sure the lower strap is around your WAIST not your hips.* Make sure it feels snug, even slightly tight. Think like being able to get one finger in if you aren't sucking in your tummy.
Then with the shoulder straps, you should be able to slide them off and back on because all they should be doing is keeping the baby from flopping back away from you. All the weight should go onto your hips via the strap around your WAIST.
*To see why, try putting your hands on your waist, on top of your hips, and then try sliding them down. Now, try putting your hands so they cup the sides of your hips and try sliding them down. You want that strap to rest on your hip bones.
Edit to add:
Before figuring this out, I was having trouble walking 15 minutes when my daughter was about the same age/size as your baby. After fixing that, I wore her for hours regularly through about 14kg. I last tried wearing her when she was about 20kg and had no problems other than her being 7 with long long legs. That's how much of a difference it is to get that lower band in the right place and tight enough.
Last edited by SecondtimeMama; 01-21-2016 at 10:46 AM.