Learning to jungle is what helped me get better about this. I've always felt more comfortable being able to have oversight over a situation, probably from growing up playing games like Final Fantasy Tactics and Fire Emblem. Until I learned to jungle I would briefly glance at the mini map once in a great while, but not even the whole thing. I was basically using it just to check on my direct surroundings instead of seeing how the entire map was looking. Once I found my niche as a jungler, there was less pressure on me to be so focused with no lane opponent to deal with, so I got better about tracking all three of the lanes' positions and relative levels of mana when I could afford to look that closely, and at the very least I can keep a note of how my teammates are doing on CS compared to their lane opponents and, if they need some help, I can give them the breathing room they need to catch back up on farm.
Countering tunnel vision isn't really something that there are any finite tips which can solve for you. It's purely up to you to ensure that you're using everything you have available. Mechanically speaking, try and peek away from the main body of your screen anytime you're in relative safety during your laning phase. For instance, if you're up against someone who only has skillshots, and there's a minion wave between you and them, you can afford a few miliseconds to glance at your minimap and see where about your team is. Scroll around the map while you're backing in a safe position, and on your way back to lane keep an eye on things until you get to the point where you need to focus on yourself again.
Avoiding tunnel vision is all about stealing the small moments it takes to keep your relative field of vision expanded. After enough practice with it, tracking the game at a glance will become second nature. I'm so used to it at this point that I can, in less than a second, take in the positions of allies and enemies and the differences in CS/kills and assists to run a quick calculation on what lanes are ahead and what lanes need help so I know where to roam, and I can do all of this while still in lane.
When an enemy is going all in on you, don't worry so much about tunnel vision because that's when you NEED to be focused on yourself more than anything else. But if someone isn't pushing on you heavily, you should be able to spare half a second to check on your mini map.
Also, get used to reacting to your peripheral vision. Even if you're entirely focusing on your own immediate area, your eyes are still picking up on changes through your periphery whether it's on the main part of the screen or on your mini map. The human brain is a wonderful thing, and it's easy to underestimate just how much processing power we have. What might at first seem like it's just instinct or a funny feeling about a situation you're in is, more often than not, actually your mind trying to pull your attention to something you've passively noticed but aren't actively focusing on. When you see the blips of champion icons show up on your map, don't resist the urge to glance down. The quarter second it takes to look at the map to see what's going on isn't going to be the death of you in lane.
