You can still save the plant. If you are indeed not green-thumbed as you claim you are, the easier choice might be buying soil meant for cacti. Most hardy varieties of plants will survive basic mistakes. I believe I have this particular plant and from what I know it is hardy, so the soil/compost does look suspect. Your other problems could be: over-watering, and improper drainage as indicated before.
Cactus plants need to be planted with gravels or gravely soil at the top, so that water does not stagnate around the roots, which more often than not are shallow and will rot from over-watering or stagnation of water. A pot with hole(s) at the bottom is ideal, and if you use a saucer, make sure that water doesn't collect and stay in the saucer.
Before repotting, I would carefully remove the soil/compost you have currently used, scrubbing with a wet paper towel to remove all traces.