Don't hesitate to write that letter today. Write more than one. Write another one tomorrow. Connect with people you haven't seen for a while. Tell them how much they mean to you. Make plans to get together.
No, a letter won't cure someone's depression or fix everyone's problems, but you have a better chance of helping someone who needs help, even if it's yourself, if you do something rather than nothing. Reach out.
Let go of the excuses.
No stationery? Write on any scrap of paper you can find, the back of a sales flyer, a paper grocery bag. There's paper all around.
No envelope? You don't have to have one. Fold your writing paper into a homemade fold-a-note; write the letter on the inside and the address on the outside. If you can, tape it closed.
No postcard? Cut the front off that box of PopTarts you had for breakfast and scribble a note on the back of it, leaving room for the address or taping the address to the front.
No stamp? Scrounge up 49 cents and buy one at your nearest post office. (It only costs 34 cents to mail a postcard in the US. Note: postcards must be no smaller than 3.5 by 5 inches and no larger than 4.25 by 6 inches for that price.) If you can't find 34 or 49 cents, hand-deliver the letter.
No one to write to? Write to a family member or an old friend. Write to your neighbor. Write to a soldier. Write to a stranger and leave the letter on a park bench or a bus seat. Sign up for Postcrossing or some other letter writing project. Browse through some of the posts on this blog for places to find penpals. I've written on the topic several times.
Don't wait. Write a letter today!