Should I keep putting money into my 1999 Honda Accord?

I have a 1999 Honda Accord that's still reliable but repairs come up from time to time.2 months ago, i coudnt open my gas tank to put gas and it turned out that the cable to open the hood and gas tank is out so I need to get it replaced. I haven't gotten that fixed since it won't effect the performance of my car.

However, when I took my car for service, i was informed that my suspension is almost out and it will cost me $500 to get it fixed since I need the entire syteem replace. They said if I don't get it fixed it will damaged my tires eventually.

So is it worth it putting money in this car? Are repairs going to keep coming up?

Yes, repairs are going to keep coming up. After any technology gets to a certain age it starts to break down from normal wear and tear. Which is why in an older car you'll have to replace a starter one week just to turn around and replace a transmission seal a few weeks later.

Decide if you'd rather keep putting more money into this 15 year old car or put (more) money into a newer car that won't break down (as much) and will be more dependable.

If you keep the car you will continue to put more and more money into it as more and more things get to their end of life. Rubber drying out, wheel bearings wearing out of true, etc…

A new car will cost you 400 or so a month - if over a year your monthly repairs end up costing more than 400/month then it may be time to throw in the towel - but if you are only putting in 1500 or 2000 a year (not including tires, brakes and regular maintenance items) then you are probably better off keeping it a few more years.
If any one tells you your entire suspension is about to go - get a second opinion - I have pushed cars for well over 150k miles without even one front wheel alignment without eating up tires.