Significant loss of power

[Zedx]

My '91 300E-24 Sportline runs beaut until I switch the Aircon on. Now, I know that's normal, I owned 25 different cars so far and was able to always "feel" the difference in RPM when it kicks in which never greatly compromised the engine's performance.
But this is ridiculous, especially if there's a hill. Pressing the accelerator to the floor makes no difference. I estimate the loss of power exceedes 50%.
Any suggestions?

I have posted this question in Australian Merc Forum and got this reply:
Are you getting cold air when it is switched on?
If not maybe the compresser is stalling because of overgassing or internal fault.
My answer:
Yes, I'm getting cold air but I switch the A/C off after a minute or two 'cause I slow the traffic down.
These problems are almost 9 months old but my situation suddenly changed for worse in a major way (finances & health) and the car problems are not on top of the list of priorities. I fear if something serious goes wrong with it I would have to become a former owner of a Merc as I live off a Disability pension.
I gave up my career, my marriage, my home, my health - and I'm over it - but giving up my Merc would be a different story!
Only the owners and lovers of this type of vehicle can understand what I mean.
The good thing is I own it outright. The bad thing is I'm not very knowledgeable asI always left car problems to the professionals to deal with... but now I can't afford them. The Merc forums are my best (and only) option... and hope.
So, to those who answer my questions (preferably in plain english) I sincerely thank!