Quick Tip: Guilt Not Welcome Here
Just let go.
Sure, sounds easier said than done but here it is. It's too late to feel guilty now. It's spent. It's over. You're not getting that money back.
If you're a woman (and many men fall into this boat as well), then you've spent money on some sort of cream, lotion or serum that didn't work for you. We're all looking for that product that will do what it promises. Sometimes it does. Often it doesn't. We can't feel bad about it. There are faaaar worse things out there that are more worthy of our focus.
If you find yourself with a bathroom full of partially used products that don't work for you, ask a friend if she would like to try it. You never know. What doesn't work for you may work for her. She may have products she can't use and you could do a little product swap. If nobody wants it, then trash it.