I've spent a lot of time discussing these things with my local scene and there's certainly no one way to play that is inherently superior. I don't think becoming offense oriented or adopting a zoning style will immediately lead you to victory if it's not something with which you're comfortable.
In competitive Pokemon there's something called STAB. Same Type Attack Bonus. Let's say Squirtle uses Water Gun. This will do more damage because Squirtle is a Water Type. Let's say somehow you teach Charizard Water Gun, he will not get this bonus. Squirtle is rewarded by using tools of his nature.
I think fighting game characters are the same way. If you're comfortable being reactive, then find a character that rewards you for using those tools instead of a character who rewards mainly rushdown or nonstop zoning. I actually happen to be an extremely reactive player--turtling has been the foundation of my entire competitive career so I understand your frustration. Sometimes zoners can smother you from afar and rushdown can smother you from up close, so just find a character with tools to combat that and beyond anything, learn your matchups. To play defensively you have to know a lot of frame data--you have to know what's safe and what isn't so you can maximize your punishes. No point playing defensively if you don't know when or how to capitalize after you've correctly blocked the mixup/frame trap.
You just need to find a character with the right defensive tools: a character that can nullify projectiles or burst through blockstrings, or a character with excellent spacing or Anti Airs. It might mean you don't always play your favorite character if you feel more at home with an opposing playstyle, but that's a choice I suppose you'd have to make. I'll give you some examples of how I've been able to a pretty successfully turtle:
MK9 - I played a ton of Mileena. From afar you could block and buffer forward forward motions in between sais until you saw a projectile, then hit X on reaction to telekick. This kept zoners very honest from full screen. You could frustrate players in the neutral with d+4 and force them to jump, in which case you would roll on reaction. She could even low profile a ton of pressure, making some rushdown tactics hard to implement against a poke-happy Mileena. You could force people to play your game in a lot of matchups. She played a great turtle game and I've had a lot of success with it against top players.
KI - I immediately loved Spinal. He can absorb specials or teleport to combat zoning, he has great long range normals for poking in the neutal and once he has skulls he can zone so effectively. Obviously Spinal has a lot more going for him with run cancel pressure and crossups, but I'm just showing how he can be very comfortable in a reactionary role. Now at some point you will have to switch gears a little and go offense (which Spinal does brilliantly), but having defense be the foundation of your game can still work.
MKX - This timing is pretty humorous because I play Sun God Kotal and he just happened to be buffed. Things have been a little rough, but with this new patch I can a) parry many projectiles in the game to help combat zoning and b) more reliably anti air with his grab. These are huge for him because it will allow him to force certain characters to come to him, which is exactly what a reactionary player wants. He still has great ranged normals for punishing jumps or runs. I'm not saying he's top or anything but as a reactionary player I have to love these tools. Plus he can buff himself with parries so if you make the right defensive read or parry a projectile and open up the opponent, he's going to bop them hard about half their life. Nice thing is the AA and Parry buffs are not exclusive to SG Kotal--you could pick any variation and play defensively.
If you are most comfortable playing defensively that may never change--it certainly hasn't for me. Just find your "Same Type Attack Bonus" character and learn your matchups. It takes work but it's very rewarding to play your way and make your style work for you. Sorry for the novel but I just happen to know exactly how you're feeling. The right character with the right tools for you is out there, just find them and start learning matchups.
