My story is a lot like yours. I had beautiful three a-b curls and they annoyed me a lot so I straightened them on a bit too high heat setting and heat damaged them. My roots and ends are curly and my curls are returning slowly, but the middle of my hair is quite limp. Here are some tips I've been using to retrieve my curlies:
One: NO HEAT! if you desperately need to straighten, there are products that you can use that will make your hair straighter, such as Super Straight by Bedhead etc. And if your looking to curl your hair, you can use bendy curlers.
Two: Cold Water. I found that if you rinse your hair in cool/cold water after you shower, it reduces frizz a bit and adds shine and moisture.
Three: Products. I use Redken Repair Shampoo, TIGI Catwalk Curl Amplifier, and It's A Ten Protective Spray. Try them! They've actually helped a lot: my hair is shinier, more controllable and my curls are definitely coming back!!!
Four: Protein Treatments: Apply eggs, olive oil, yogurt, Mayo and bananas to your hair, put a shower cap over it and leave in for atleast fifteen minutes to add protein to your hair. This is important since your hair is practically made of protein and it will probably add curls and bounce to your hair.
Lastly, trim!!! If you have severe damage, you're going to want to cut it off. If length is important to you, you can trim an inch or two off every few months, or you can do a big chop to get rid of most of the damage all at once.
This stuff worked for me! Try it!