It is very easy to use things without understanding them. I'm guessing there may be some of that going on based on what you say. If so, a good technique to force yourself out of that box is to first imitate something directly without even trying to adapt it.
As an example, this is a technique for teaching how to learn different poetry styles. Take an existing poem of the style you want to learn, then start substituting words and phrases exactly 1:1 until you've changed the meaning to something else. Same syllables and emphasis every time. The finished "poem" is going to be dreck. It doesn't even rise to the level of parody. So with the goal of producing something useful, this is a terrible technique. However, doing this forces the person to pay attention to the technical aspects of the work--meter, rhyme, alliteration, structure, etc. In other words, it makes you really study the original in a systematic way.
That doesn't help any in integrating multiple things into a viable whole. However, if you are already adapting existing things to your table, then you've already got some skills there.
So try the same thing with the sandbox elements, however you get them, from your existing sources, or advice, or discussion here, or whatever. Take a simple wandering monster table that you like in an existing work. Translate it 1:1 directly into a wandering monster table for another area (even another existing one, not your own). The only goal is to keep the structure but change the affects. Make it more or less powerful. Change the ecosystem represented. Make it for a different game. Anything.
That's a crude example that likely you've already done without even thinking about it as a substitution technique. Well, the same thing works on "small village with 3-6 ruins or other interesting adventure sites within a few days travel" or any other game structure you can imagine. Imitate enough, you start to really understand the purpose of the structure, and then integrating into your own things becomes easy.