Hello. I Was Hoping You Would Be Able To Answer

Hello. I was hoping you would be able to answer my question regarding my diet. For about a year now, I have been living off of an 800 daily calorie limit. I first put the diet into effect when I wasn't in a very healthy mindset and had the goal of quick weight loss. For about up until 2 months ago, the diet worked and I lost a decent amount. However, recently I believe my body has become accustomed to this limit and instead of maintaining or losing weight, I have been gaining. This is without any change to diet or exercise. I want to raise my daily calorie intake to a normal and healthy amount again, but I do not know how to do it without immediately gaining all the weight back. Is there any way to slowly nurse my metabolism back into normalcy over time?