I can sympathise with you being concerned about the time/effort that changing the entire design of a site; it's all too easy to be afraid of change.
I think that you are hurting yourself by not taking the advice of so many people on here, and I'd like to add to the chorus of people singing from the same hymsheet. You should promise yourself to change one of these sites completely and then see how you feel after that - you may find that things you were concerned about were easier than you thought (and other seemingly simple things may turn out to be a nightmare and things to avoid later).
As it stands you are literally wasting time that could be better spent once you bite the bullet and change your modus operandi. Every small change that currently takes you minutes/hours could instead be taking you seconds/minutes if you were to invest time in updating your skillset/changing your mindset.
I also resist change, and I still use tables for many things, but I use CSS for many things where it's better. If you run dozens of sites then you really should have a method of working which separates the look of a site from the content. As it stands it seems to be the case that you are held back by the very thing that you don't want to change.
It might be a good idea for you to list your skills so that people can see what things may help you, suggesting things that you may learn easily based on current skills.
I think that you should resist creating more sites until you have streamlined your update/creation process (and improved the ability for you to update site design).
You don't need to use something like wordpress, if you have even basic PHP and Database skills along with the smallest helping of CSS you may be able to dramatically increase your productivity and profitability whilst easing your stress about managing many sites.
Please, please take the advice of people seriously - they are trying to help. As for the fact you have made sales; you probably could have made a lot more with a different design. Expectations are getting higher and higher - you are a decade behind and falling back - you might be currently catering to a very small percentage of people who still find your site credibility high enough (as your design calls that into question).
Ask yourself this:
Given the choice of 2 taxis for a journey, which you know will be priced very similarly or even identically, would you choose to ride in the 1 year old new looking, scratch-free taxi or would you go for the 12 year old taxi that's looking worse for wear? Which of those would you expect to be more likely to break down? If you were asked which operator was more likely to be around in a year, who would you say?
Your site is a that clapped out taxi when it comes to what image it projects about the person/company behind it. Some people will use it, but most will avoid it as they know there is plenty of choice (Google will direct them to competitors with a shiny new fleet).
The internet can be a brutal place when it comes to first impressions. Although you might not like to hear it; to me, and probably many experienced web users, your site is saying that you are a bedroom operator (that may not be around tomorrow), that you do not care about your own site (so why should you care about anything else about the quality of your operation) and that you don't understand technology (which is a bad thing when the content is any way technical).
The Good News:
Sometimes you can see incredible results by investing a (relatively) small amount of time. I changed the design of a site and saw a huge uptick in revenue for the same amount of visitors - I had avoided it as I thought I knew best, I eventually tested something after being pursuaded and found my thoughts were wrong. Be prepared to prove yourself wrong, always challenge yourself - do design specific A/B testing (this is easy when you separate design from content). I am not a designer (although I did the original design for the site in question) so I brought in assistance, it cost a little (things can be very cheap indeed) but it literally has paid off more than one thousand-fold (that's not revenue going up by that, it's the extra revenue generated by the change divided by the cost of the outside help).
Cost should not be an issue, if you cannot afford to buy in help then it's just further proof that you need to change. If you need to sell a site to be able to change the others then I think you should do that.
