Answer: Your discomfort with these situations is so common. And for those who are handy with reassuring words, it's often stressful when it comes to "planning out something they need to be done." We all have our weaknesses.
So, acknowledge those weaknesses. Since it sounds as if you're referring mostly to situations with a partner or parent, use your close relationships as a chance to be candid. It's difficult and awkward to form the words, yes, so save them for a neutral time, i.e., when no one's asking for your support. Maybe say: "You recently were upset about X, and I felt bad that I didn't have words to help you feel better. I'm more comfortable doing something to help, like coming up with money or a plan."
Next, listen carefully to the person's response. There's an excellent chance your nearest-and-dearest know this about you, possibly better than you do, and aren't really asking you to be anyone beyond who you are.
It could also be that your nearest-and-dearest are as frustrated as you are by your reassurance paralysis, and will be grateful just that you asked about ways you can help them. Or, perhaps they have observed something only an outsider can see, something that you can apply to improve your dynamic and ability to communicate. That's why this part is all about listening - you want to hear specifically what they're asking of you, as well as their insights on why you struggle to provide it.
The best part about this plan is that you don't have to repeat it with everyone who has ever leaned on you in a vulnerable moment. Use these conversations with your innermost circle to get a general idea how to conduct yourself with others - even if it means admitting to someone who leans on you, "I wish I could be of more help. I'm kind of lost in these situations." Letting them know you want to help is better than vanishing, which is what so many people do in the presence of pain.
E-mail Carolyn Hax at firstname.lastname@example.org, follow her on Facebook at www.facebook.com/carolyn.hax or chat with her online at noon each Friday at www.washingtonpost.com.