The reason you lose

The general reason why you lose a team fight or a 1v1 is because you have the mentality of fear. When the number one thing that comes to your mind in a fight scenario is, "how do I get out of this safely?" you'll most likely lose the fight. You're not focused on the fight, you're focused on your escape plan. Remember, it's a team game, if you decide to start running away from a team fight, and not holding it until the last fight, how are you gonna come out as the victor? There is winning in failure, and that is when you've tried your best.

Unless you're facing your hard counter, you don't have a reason to be scare to 1v1 or harass your opponent. Every time you turn your back against your opponent, he'll take a jab at you. This is how you generally lose in lane.

A way to remedy this mentality is, when you're up against your opponent(s), tell yourself, "I'm going to go in an execute this combo (as a practice because nothing else beats practicing on real PvP)", "What am I gonna do next/after I initiate a fight?", "How can I position myself better so I can take down the front line?", etc.

Not, "Oh shit, we gonna lose, which path am I gonna take to get out safely?" Generally, the mentality of "we're going to lose", will make you lose because you already have your mind set on it. Why do you think that in every single competition, people give encouragements and positive advice, leaving a positive atmosphere, telling each other "WE CAN DO IT", and even though you do come out as the loser, you can tell yourself you've given all you've got.

Stop being that down syndrome kid in your match that goes "we're gonna lose".