That half -blood knot you are tying is the same thing as a standard (ie un-improved) clinch knot. Although it never breaks, it basically sucks as a terminal tie for the very reason you stated.... it SLIPS and comes untied each and every time!

That's why the improved clinch knot was invented. simply slip that tag end back thru the big loop and pull on the standing end (mainline) to tighten the knot. If you have tied it correctly, the tag end should stand out at 90 degrees to the stack of coils, sandwiched between the first and second coils. That's what traps the tag end from slipping back thru the knot.

BTW even the improved clinch is a pretty lousy knot from the standpoint of break strength... only 75-85% depending on the brand of line. It's main advantage is it can be tied in about 10 seconds even if blind-folded. Ditto for the uni-knot or Duncan loop.