There are two ways to do it live without a high-end switcher with a chromakey module:
1) two tvs, play back your background to one with yourself on the other. IF you want, you can get two tvs the same size, two pieces of glass, lower the brightness on the top tv, and place the glass at a 45 degree angle out in front of each set (back the lower piece of glass with something black, cover the 'holes' from the top one in black, maybe put black around the whole thing) so you have a teleprompter style setup*
2) get a cheap switcher [yes, cheap is a relative term, this will still cost $$ and you may need a TBC] and simply superimpose your image over the other (it isn't a chromakey but you will see where you lie)
*This suggestion is based upon theory. I have not tried this.