Your breasts are still there, aren't they? There aren't any signs of an explosion? Then it's a fair bet to say they haven't been destroyed.
Whoever told you that deserves a good smack upside the head and/or some counseling. Your breasts have been
changed by pregnancy. There are any number of things that
change our breasts, including pregnancy, aging, weight loss, weight gain, etc. No one should expect size, appearance, location, etc to stay static throughout life. Humans don't work that way.
Of course, I am certain you know this, but I am telling it to you anyway, so you know that there's nothing
wrong with how your breasts look.
It's a bra issue, not a breast issue. Cotton, alone, doesn't give the greatest support to begin with. And if you're like me & only have one or two bras, then you're washing them a lot & that's going to degrade them as well. Also, if your babe is a strong eater of solids already, then you will naturally be producing less (which is just fine too!), and your breasts will have shrunken some, & that can give you a saggy look because your bra doesn't fit perfectly anymore.
By all means, you can buy an underwire, just be certain it fits properly. If you're not wearing anything above a DD right now, you'll probably be able to find bras in your size in the stores without too much trouble (heck, I'm wearing an H right now, & I've been able to buy bras in person for the first time in seven or 8 years). Either measure yourself or get someone knowledgeable to do it for you (I have heard bad things about the gals at VS). Then try on not only that size but the next cup size up. If you can find a properly-fitting & comfortable
normal bra, you can use it instead of a nursing bra by just pulling the cup up or down (I've only been able to pull it up with an underwire). You may well be able to find some really pretty bras, like demi-cup, that are easy to nurse in.
If you don't want to do that, I'd suggest buying a soft-cup bra with some Lycra in it. Many sports bras fit this bill. Just try it on and see how you look in it.