Wow I completely missed the KOS gear part - I must have automagically translated it into RoK gear instead.
Each expansion builds on the last. It's part of the progression of the game. Progression isn't just about raiding and quite frankly, it has to be that way. If they tuned TSO to what people were wearing in KoS which was two expansions ago, anyone with EoF or RoK gear would blow through the content without any challenge whatsoever.
You need to step back and think about it logically. If you are expecting to do TSO 75+ instances in KoS gear, you're a fool. It would be fine for a L50 to start gearing themselves up in TSO instances because they are scaled for L50 and what a L50 on average would be wearing, and there is a reason why not all instances scale down to L50.
I wouldn't group you if I noticed you were in KoS gear and I'll group just about anyone who hasn't already proven themselves a poor or unpleasant player. You have unreasonable expectations if you think at L80 (which I will assume you are at since you said mythicals were an issue) that you can do it with gear from two expansions ago.
Maybe you should try mentoring down and doing some of the scaled instances with L60ish groups - or try progressing through the previous content to gear yourself up a bit.
And just a suggestion as one healer to another - get ProfitUI if you at all can and use it. It makes healing the rest of your group much easier because you can keep your target on the tank while doing so. If DPS does not occasionally gank aggro, they are not trying hard enough and your job as a healer is to keep them alive unless doing so will cause you to lose the tank.