welcome to my world
I'm 5'7'' so everyone I spar with has a longer reach than me. I agree that you have to fight inside your opponents reach, his shots will be less powerful when your inside. It will feel dumb at first especially if you are used to punishing people from the outside but you will get used to it. Don't be afraid to take some shots inside there(you will get hit I guarantee it) just stay cool. This is how I usually approach a fight with someone a lot taller then me, I will come in working my jab, though its not connecting its helping me feel out his reach compared to mine. If he refuses to initiate an offensive attack I will jab punch combo to get his hands up, then move inside work his body then on my way out I will try to set up an uppercut/left hook. This doesn't make for an instant knockout kind of fight but instead if you can move in and out and effectively land strikes on your opponent he will get frustrated and make mistakes that YOU NEED TO CAPOTOLIZE ON! Watch for him to start dropping his hands more, or throwing wild punches, or watch for a lazy jab. If you catch him doing any of those things it is time to make him pay!
Good luck and I hope this helps