DDT is a useful insecticide, especially when used strategically, i.e. on bed mosquito nets. However, DDT wasn't used strategically in the 1950s: it was broadcast sprayed on cotton fields etc. The result was that most of the target pests developed DDT resistance. DDT itself is very low in toxicity, but DDT manufacture (inevitably) makes dioxins and chlorofurans which have known environmental effects at very low concentrations- known, not suspected, as the mechanisms by which they do their damage are understood. The same goes for pentachlorophenol, PCBs, and numerous other very useful 1950s chemicals- the problem wasn't so much with the pure compound as with the co-contaminants, and with what you get when you improperly dispose of them.
Total bans of materials such as DDT are actually dangerous: they put people's lives needlessly at risk. But it's very hard to put in place regulations with effect worldwide that allow proper, strategic applications of these compounds. It's also VERY hard to regulate the manufacture, use and disposal of these materials such that the truly harmful contaminants are either never produced, or removed and disposed of properly.
As to CFCs: the trouble wasn't in the use as refrigerants, where they were very useful and safe- it was the use as aerosol propellants, and in the venting of refrigerants into the atmosphere when the devices leak or are scrapped- something that can be reduced but not eliminated unless you stop using them at all. Chlorine atoms in the upper atmosphere catalytically destroy ozone, with turnover numbers in the 100,000 range- that too has been demonstrated by good, solid testing and is not in dispute. The reason CFCs were a problem was that these molecules were photostable enough that they needed hard near-vacuum UV- of the same wavelength range necessary to make ozone from oxygen- to photolyze, so they had the ability to survive the conditions in the lower atmosphere long enough to get up high enough to do their damage. The "self healing" of the ozone layer has little to do with the regeneration of ozone itself, which is of course happening all the time in the upper atmosphere- it has to do with the (low) rate of processes which remove chlorine atoms from the upper atmosphere once they're generated there.
As to CO2, we need to stop wasting fossil carbon as a fuel to the extent that we're doing now. We have nearly doubled the atmospheric CO2 concentration as a result of burning fossil carbon- that is a fact based on measurements. CO2 is a greenhouse gas- yes, that too is a fact- only the extent of its effect is in dispute. Its mass emission acidifies the upper layers of the oceans- again, fact, not in dispute except for the extent of harm that can be expected from it. These risks of harm are only two of many reasons to price fossil carbon for fuels use in such a way that we treat it as what it is: a precious, finite resource that is VERY difficult to substitute for in just about every other use we put it to OTHER than its use as a fuel. Most of the non-fuels uses for fossil carbon can be considered just as a "parking place" for the material: at the end of their useful life, they can be either recycled OR used as a fuel.
