Dont worry about opening theory.
You usally lose on rating 1000 when you give away pawns or pieces for free or hang mate. So try to not give away anything for free. The 2 thing that help me the most is to do puzzle and to analyze the games after you played them. Try to do some puzzle and analyze the games between each game and you will see that your rating soon will start to climb.
When you analyze your game you can click on keymoment and there replay your misstake. That is what i usally do.
Also i do notice that you have not played much, you gotta play way more. Keep playing 
I recently got back into chess having not played in almost year. I am currently 946 rated in rapid I want to reach 1000 rating however I have found that a lot of my opponents are considerably better than me. Does anybody have any tips for improving? I fear it could be my lack of opening as I only know the London system, Caro khan and am currently learning the slav defence.