First, it would be good for you to exhibit an improved attitude. None of us here claim to be any kind of hot shot, but instead make a sincere effort to try and help someone out.
Secondly, it is unfortunate when a piece of software is no longer supported with a new OS, but typically, there are viable alternatives. I also used to use Eudora for my EMail needs, but given that I use Word and Excel also, I just "upgraded" to Office 2008 in initially, and subsequently to Office 2011. The Office package includes the EMail program Outlook, and from what I remember, it was a breeze to import everything from Eudora to Outlook.
Fortunately, for more recent OS upgrades, for all the third party software I use, it either continued to work fine, or the company had upgrades available. When I upgraded from Yosemite to El Capitan, I did need to upgrade 6 critical programs, and it went well.
For your situation, as long as your use of the machine is "simple", and given that Yosemite is not an "old" OS, you need to stay where you are. That is, if all you use your machine for is to check EMail, browse the web, and fire off an occasional document, it is not necessary for you to upgrade.