Buy a 6 pack of redbull and just think to yourself, the quicker you finish the design, the better you will feel

Also think of the money

Sounds like you need to reorganize your business practice and include a contract with work you do. Offering cheaper pricing for your time and effort does not mean that you should accept poor work, daily modifications and anything else that hinders your efficiency.

Limit the amount of modifications per project and charge additional for x amount of hours or modifications required.
Require at least some organizations with PSDs, include fonts, and ONLY a PSD. Not PNGs or JPEGs (unless its like a Fireworks layered PNG which is possible, right?)

If the client gets to the point where you are fed up with their crap, sometimes money isn't worth it. Tell them you can't fulfill their work due to personal reasons and promptly end the relationship.

Just explain to them (calmly and professionally) that the file they sent you is inadequate, and that they should talk to the original designer to fix the problems you have.
If you are strapped for cash, and can't afford to turn down good business, then It's best to just let them know that in future projects, to group / name layers, don't rasterized text..etc..etc and just make the changes yourself (this time)
It will make the client feel you are going above and beyond for them and that you care for the final outcome.

Well said Aaron, sometimes its hard to keep your cool but its best to keep it professional as you are running a business and providing a service.

Honestly, from a long time of working for cheap for clients that thought they deserved the world for $50 I finally moved away from that kind of stuff. The gaming community, whether it's expensive or cheap work, is full of people like that, it can ruin your reputation as well, trust me I know first hand. I do not do work online for gaming sites anymore unless I know the person personally, it's just completely not worth it at all in my mind.

I charge for time worked. If the customer is "tough" in that they have a lot of changes to the requirements or feel things need to be reworked or improved, that's fine with me. I like a customer that is involved in their project rather than one that gives no feedback. And when customers start to get "tough", I keep them talking by asking lots of questions, and make sure they receive an invoice for the time, so there is no confusion about the commitment they are receiving from me.