Destroying your embryo (though I totally understand why you feel you can't) is not destroying your baby. There are umpteen fertilised ova which are washed away during menstrual cycles because they didn't implant - and this happens for ALL women - not just women who find it difficult to conceive.
As I understand it, when embryos are implanted they usually put in two or three to ensure at least one viable foetus. All you know is that this is a healthy embryo - not that it will necessarily lead to a healthy pregnancy (or any pregnancy).
As you get older, even in our technological age, each pregnancy puts a greater toll on YOUR body.
What if you are very, very ill and can't even properly care for the little one you have?
What if you miscarry at a time when you are happy and joyful and filled with hope and anticipation for this coming baby?
Or - God forbid - what if you longed for baby dies or is seriously damaged during the birth (anoxia etc). Or you do? Who then looks after the beautiful baby you already have?
A longing for a "last, bonus" baby is very common in women when they start to reach the end of their childbearing years - I had it myself. (Mine started when my DH had his vasectomy and even though we did not want further children, every month I would hope that I would be late and that the "V "had reversed itself.). It is a very natural and understandable emotion.
Your hormones are still jumbled from your other baby. Give yourself a chance to let them settle.
Perhaps (I don't know how you feel about these things) you could speak to your priest and ask if you could have a small service of mourning for this last embryo, and then let him go in whatever way you are most comfortable with - either to a child's couple, or destruction (what a terrible word!). In fact, many, if not all, hospitals have their own services for babies who have died, were still born, miscarried etc, just because they know how devastating a loss it is to the parents. Maybe if you could think of him as a "lost twin" to your healthy baby, it would be easier to let him go.
Whatever happens - please don't go ahead without your DH's wholehearted agreement. It would be a terrible betrayal of trust. Give yourself at least a year to allow your body to recover from your most recent pregnancy - physically and emotionally - and see how you feel then. Yes - you will be a year older, but you will also be thinking more clearly and will be in a better position to know what you - as opposed to your hormonal imbalances - really want.
Whatever you and your DH decide, I wish you every joy.