From fat to fit to fat and back again

I'm Shelly. My weight has been a lifelong struggle. I've been successful in the past at losing the weight, but have never managed to keep it off... I've weighed as little as 115, and as much as 240. I'm tired of being the fat friend and knowing my body is being abused being this big. My problem is the cycle of losing and gaining it back. It's so discouraging to never seem to be able to maintain. Looking for people who will encourage and motivate me.