Middle game advantage lost towards the end of the game.

I don't have thorough knowledge of openings ( 3 to 4 mains lines I know), but I know opening principles and I manage somehow. Towards the latter part of the middle game I am usually in a better position but as the middle game gets to end game I loose my advantage. When the board opens up and the pieces are spread out, it becomes difficult for me to find the best moves.

What should I do to improve my game. I am stuck in 1200 - 1400 range for many years now.

Before you can even dream of improvin you first need to (ALWAYS) be able to express the chess problems you're experiencing, you basically wrote "I don't have excellent opening theory" but it doesn't say anything really, but what if you wrote this instead:

"I can't seem to i prøve my ratings, i have a very hard time converting winning positions, I play this one opening that I'm very comfortable and familiar with, I often quickly earn a winning advantage early on due to my pretty solid positional play, but then when situations arise when there's too many tactics available I seem to get overwhelmed and a bit confused and before soon I'm playing a hopeless endgame with massive material disadvantage. My endgames are pretty good as well, I know all of the basics by heart, Lucena, Philidor etc"

I've only been playing (and studying) really over the last 6 months and seems that I've already heard everyone (that is someone in the Chess world)

Say that I shpuons

That I shouldn't really be studying and trying to understand opening theory until I first have a really good understanding of most if not all things chess. Since I'm not even 10% there at the moment I simply just know that regardless how many lines I memorize in how many openings it won't matter diddly squat when I'm still just me during the middlegame and endgame. Personally I don't consoder playing studying, for me studying is when I predetermined what I'm gpi g to learn and then I study that until I have learned it and then I try using that in a real game.

I apologize to readers fpe this mess I don't know why I can't edit or even delete posts, I'm on the app on my phone. If there's a better way of using the forum please let me know. Cheers

Are you playing OTB or online?

I would consider getting a coach to look at your games and pinpoint exactly what you are doing wrong and how to fix it as soon as possible. Here to help if interested.

If not, I'm sure there are a lot of people who will give you free feedback. It's best you post some positions where you are not sure what to do and ask people to help you find a plan.

You need to learn endgames. If you don't know how to cash in your middlegame advantage it may be that you are overestimating your "advantage" and your opponent actually has the better endgame chances. Even a basic knowledge of what makes a winnable ending will help you judge when/whether to simplify the position.