Wow! I just came across this! I am so sorry Joy! You seem like a very caring and capable person. Don't give up just yet. I agree with the others suggestions. Take a step back, and take a deep breath. It isn't your fault at all. We have all been there at one time or another.
Another option would be to take the remaining fish back to your LFS, leave the tank fallow and start over. Learn as much as you can. So you can do most of it your self, if water changes are too difficult to do yourself pay a maintenance company for that, but do testing etc your self. Personally I have never used a maintenance company. I trust no one but myself with my tank....not even my husband....lol....than if I screw something up it is all on me.
Don't panic and don't beat yourself up. It will be ok. This hobby is incredibly rewarding, and on the flip side it can be incredibly frustrating at times too. The good outweighs the bad though. Good luck and please keep us posted!