If you feel that your dentist is advising you to have unnecessary work done just to get more money out or you, get a second opinion. I would suggest that your dentist is only advising what he feels you need to have done. However, a second opinion will either confirm that fact or give you another way to solve your issues that will hopefully be more affordable.
Try explaing to the dentist that you do not have much money and see if they can just focus on what you have to have done now. Then do the rest a little at a time so you can afford to have the work done.
You may also want to look into getting a dental discount plan since you stated that you can not afford dental insurance. Most discount plans have a low monthly cost and they will help you off set your dental cost.
Here is a link for that if you want to look into doing that