I understand your frustration and I see it everyday with assassins.
I had this kata 0/5/0, 2 lvl behind and half my cs today that I honestly hard outplayed in lane. Yet, she remained a threat the whole game. Eventually she 100-0 me once but also got kills here and there even if she was super behind. It didn't matter at the end because we were super ahead anyway.
I assume you had no flash and no heal and I assume your support was brain afk. I assume your midlaner missplayed and was probably coming back from base. There's no way she would've ganked unless : 1°) she didn't force her opponent to back while remaiming healthy or outplayed your midlane by backing first to roam. 2°) your midlane or yourself haven't warded deep enough.
The game has become less forgiving because of the amount of damage which pretty much makes the assassins have it easy. I know you're Plat 3 and you're probably pretty good at the game but it doens't mean you don't make mistakes and I'm pretty sure I'm correct on my assumptions.