This is definitely something that happens for most players. Not only is it fairly instinctive, since most everyone is right or left handed and has a stronger hand/arm because of it, but it's also something that can be hard to break out of if you've been playing for a bit, as it is a habit that has been formed. I would recommend trying to exaggerate what you're trying to change. Try playing something relatively simple for both hands and play the left hand as quiet as you can and the right hand fairly loud. This should help your brain break the habit a little and once you feel more comfortable doing that, reduce the exaggeration, bringing the two hands to a more similar volume. As this gets easier for you, try making the parts for either hand more complicated, starting off exaggerated and moving toward a balance.
For the one finger, I don't have quite as good advice but I'd recommend focusing on the one hand, as opposed to playing with both while trying to fix the issue. You might want to try playing with that finger and just one other at a time and focus on getting the same volume out of them. You're probably just having that finger closer to the keys in general, so when they all come down, that one hits first and hardest, so you will probably notice that as you work through using different fingers. You can try holding that finger further up than the others so that it hits at the same time, or you could try extending that finger further back on the key you are hitting. The further up the key you are, the harder it is to get the note to sound, due to basic leverage, so it should naturally be quieter. The only real issue with that approach is that you don't always have the option to do that since not all chord voicings allow for you to choose where on the key you hit.
On the whole, you're needing to get your brain to break a habit, so you have to actively seek the means of doing so through mindful practice. You're probably currently doing things without actively thinking about them, so by actively thinking about it and using the right approach for you, you can retrain your brain and then won't have to think about it once you've broken the habit.