You can try hedging. To understand what I mean, make a quick research on this please. It's a simple thing. You open an opposite position in the market, so that you make your value constant at that time you opened the position.
It is bitcoin, mate, no matter what you do, it will keep on increasing and increasing day by day. If you don't rrally want it to increase, then don't convert it to bitcoin, just keep it in fiat, so in that way, what you want could possibly happen. It is just impossible what you wanted to happen since we are talking about bitcoin to the point I don't know what to say.