Whipping double cream with amaretto

Whipped cream - as in, pure cream that's been whipped - isn't stable. Even just left on its own for a long enough time, it will collapse without a stabilizer like gelatin, xanthan gum, and/or dextrose.

You added water and alcohol, which is pretty much the opposite of what it needs. I'm hardly surprised that it collapsed. Most whippable items will fail to whip if you add more than a tiny bit of water. You might have been OK if you'd used a stabilizer, but even then, I wouldn't count on it.

Chantilly cream is a mix of 3 parts whipping cream to 1 part pastry cream (which doesn't whip, but is thickened with eggs). Because it is so much more stable, it can accept much more in terms of flavouring. It takes much longer to make but would probably have handled the amount of amaretto that you added.

But a much better solution would be to use almond extract instead, which imparts far more almond flavour with far less liquid.

A lot of people make the mistake of thinking that flavoured treats can be easily made from liqueurs. Most of the time, liqueurs have far too much liquid. I once, long ago, made the mistake of trying this with truffle ganache, which is far more forgiving than whipped cream - it got way too soft and still had hardly any of the flavour.

If you're able to adjust the water in a recipe then you can use liqueurs, but usually you're better off buying some concentrated oils or extracts, which tend to pack as much punch in a teaspoon than a full cup of the liqueur. Some are easily found in grocery stores (almond extract, mint, etc.) and others you can get from specialty vendors like Lor'Ann.

Once you whip it (and I suggest you add a hardening agent to make sure), add whatever you want in it and mix slowly with a spoon. Never did it with amaretto, but Nutella (and similar chocolate spreads), walnuts, hazelnuts, coconuts, Baileys, coconut rum and mixes of the above all worked.