I was in the same situation as you when I decided I might want to go through AFF. Did a tandem in 2004 then again in 2007. Started AFF in 2009 and got my license in 2010, I did not do another tandem before AFF.
If I were you I would take a look at the online ground school (the URL below) to educate yourself on the things you need to know in order to learn to skydive. If it is still something you think you might want to do go sign up for actual ground school at the nearest drop zone. After ground school your instructor(s) should have a good read on you and whether or not they think you should do a tandem before your first AFF jump. Also ask them about it and what they did. Everyone is different and progresses at their own rate.
Skydiving is expensive, there is no way around it. It would definitely be cheaper to go straight to AFF without doing another tandem. Before you make that decision, however, be sure to ask yourself "is my life less important than the money it will take to ensure my safety?" If you have to do another tandem, so what, it will just prepare you more for your first AFF.
Also, it is very normal to be nervous and terrified for your first AFF jump so expect to be when you are in the door
http://skydiveschool.org/#/cover/