yahoo messenger doesn't show what i write?

If you you cannot see text in Instant Messages at all, not even your own after you click Send then read below. Yahoo does have a limit on the number of characters that can be posted at once in an IM, I am not sure what that limit is though.

CAN’T SEE TEXT IN INSTANT MESSAGES? 
If you cannot see text in instant messages, not even your own after you click Send or Press Enter, this is an issue that is cropping up more and more. If just a new problem try exiting Yahoo Messenger and then relaunching it. Even try rebooting your computer. If both those fail to solve your problem then try the suggestions below. This is actually an Internet Explorer scripting problem. Yahoo Messenger relies on Internet Explorer to work even if you have a different default browser. So when IE has a problem, Yahoo Messenger can as well.

1) The first thing to check is to make sure you have scripting enabled in Internet Explorer. Internet Explorer is integrated into Yahoo Messenger so things that go wrong with IE can affect Yahoo Messenger. Here is the yahoo help page about that:http://help.yahoo.com/l/us/yahoo/messeng…

2) In the Start Menu click on Run. Type, without the quotes, “regsvr32 jscript.dll” and then click OK. Next, repeat the process but this time type in the Run box “regsvr32 vbscript.dll” and click OK. Please note, if you are using Windows Vista temporarily disable the User Account Control (UAC). You can easily turn it back on. On the Control Panel (Category View) choose User Accounts and Family Safety, next choose User Accounts, and then click Turn User Account Control on or off.

3) If the above does not fix your problem and you are using Windows XP, you can update your Windows Script to version 5.7. In my experience helping others, this is the step that has helped the most people. Exit all browser windows and chat programs including Yahoo Messenger before installing. Here is link from Microsoft to update that:
http://www.microsoft.com/Downloads/detai…

Or you can do an internet search for Windows Script 5.7 and choose the Microsoft link.
This version of Window Script is already included with Vista so you would not use that link. There is also a Windows Script 5.7 released for Windows 2000 and you can find that here:
http://www.microsoft.com/downloads/detai…