Can i keep my car forever and just keep fixing it up ?

It will be harder to do if it was a kind of rare model, and it is my opinion that the NEWER a car is the HARDER it is to work on.  After 25 years of age the car no longer has to pass emissions controls standards, just safety.  As the engine wears, that emissions control standard gets harder and harder to hit, so that at around year 22 that standard is almost impossible to hit without spending big bucks.

  I view it like this:  Imagine that there was a law that in order to keep your dog, it (the dog ) had to jump over a 2 1/2 foot (76cm) tall fence every year, and if he couldn't jump over that fence, they wouldn't necessarily take him out and shoot him, you just couldn't buy medication for him, or take him outside the house.  There was one exception to the rule however, if your dog was 12 years old or older they didn't have to jump over the rail every year.  Well.... you see, most dogs get old and arthriticy enough by the time they are ten, that that exception isn't going to do most of them a lot of good.

  In a similar manner I think quite a few cars are sitting in the salvage yards because the owner was looking at a $4,000 bill to fix something that didn't affect the drivability of the car, but it DID keep the car from passing emissions controls inspection, and they didn't have the luxury of being able to hide the car in a barn for three years until it didn't matter anymore.

  In a way it will actually get EASIER to maintain your car after it turns 25 years old (at least for a while it will until parts get scarce) because all the repairs will be a little more straight forward.  A lot of people are complaining about how after a while you will sink more money into the car than it is worth, and I guess they keep thinking in terms of resale value.  I think of it almost as a cost/benefit ratio.  When I complained about how much the C.V. axles cost to replace, the mechanic said,"Well, it's cheaper than making payments on a new car." and, you know, he was right. 

  I just did the math and the purchase price and the several big repairs spread out over 17 years, and it has cost me a little over $176.00 a year not including insurance and gasoline.  I mean that's down around the price of riding the bus.  It's really going to help your bottom line if you can really get into the grease monkey stuff.  I pulled the wheel off and re-did the C.V. boots, cleaned and repacked the inboard and outboard C.V. joints with fresh molly grease and stuck it all together right, and I realized that I just did $350.00 worth of work on my own car!

sure, if you don't feel that is wasting your money.

In theory yes, but in practice you will come to a point where your car is worth say £500 and it will need say £700 spending on it to keep in roadworthy. If you spend this £700 you will then still have a car worth about £600 and in a years time you may still need to spend yet another £700 to keep your car roadworthy - there are many parts on a car and if your car has done 100,000 miles plus then most of these parts will be significantly worn and likely to break down. At some point you will find the cost of repairs is just too high.

It's always cheaper to fix than buying new. 

The question comes: If you have changed every car part, is this car still "your car"?

I wish I had pictures of my grandads, he has perfect condition classic cars locked away he takes them all across the world for shows. I'm not saying you'll keep the original engine ect, but you can keep your car by putting in some money and dedication. You also don't want to. Try land rovers, sports cars ect. Find what's for you

As long as you have three things:

Money: Over time, the cost to repair the car will be more than the car itself is worth

Time: To devote to the car and all its upkeep...which will keep climbing as it ages further

Parts: As long as spare parts are available for your make/model. 

I thought I could do that, but after 22 years and 285,000 miles parts were no longer available. That was a Mitsubishi and they never support their products so well. Other makers, make parts for ages and ages. Consider what car you're trying to drive forever. You might find a machinist who can make rare parts, but that will cost $$$$$$$.

observably you have never worked on a car have you?

Nothing lasts forever. Not even you. In Havana, Cuba they are still driving cars from the Forties and Fifties. So you can keep going.