Hi Betty,
As you go into this, I would start with the mindset that you may ruin the cross.
The first thing I would try is to sand off the raised heart in the center. I have a belt grinder jewelry sander that would make that process much easier than doing it by hand. If you do not have one, doing it by hand will be quite tedious. A dremel tool with a sanding attachment would likely work too.
You could try applying some heat to the pendant to see if the two parts come apart. Perhaps a warm oven is all it would take for the heart to come off. Assuming they are two separate pieces, you might get lucky and some heat will melt the adhesive holding the two parts together.
If that doesn’t work, you could try soaking the piece with some acetone or denatured alcohol, but you will likely damage the cross too.
Whatever you try, would you mind letting us know what works?