Hi Sasuke,
You are talking about chemical energy, i.e. energy stored within chemical bonds. And you are talking about potential energy, energy stored within a spring. Both of these are already energy, and matter is by no means converted into energy in order to use that energy.
When you eat the sugar in a plant, you are not converting the matter into energy, but utilizing the chemical energy stored within the molecular bonds (I'm not a chemist or a scientist for that matter, but this is how I understand it works). The matter stays matter.
In a nuclear reaction, OTH, atomic nuclei are split into smaller nuclei. Releasing energy in the process, and losing mass proportionally according to E=mc^2. This would have been a better example for you to use. However, although matter is lost and energy is released, it does not mean that matter is "stored energy", though you are certainly free to think of it that way.
I can see how energy and matter could both be manifestations of fluctuating fields. Electromagnetic radiation, light, for instance, is a fluctuation in a magnetic and an electric field. (I'm not sure if I'm using the proper lingo here). If I remember correctly, all particles behave like (are?) waves to some extent. Electrons passing through double slits will create an interference pattern just like light does.
Respectfully,
-Meldinoor
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