First time kegging trouble!

I bought a pin lock corny keg and decided to put a batch of Oktoberfest in it. Heres the facts:

1) I didnt' have a lot to spend so I got a portable CO2 injector and some 16g Co2 annisters.

2) I naturally carbonated the five gallons of beer with 1/3 cup of corn sugar, then filled the headspace with CO2 using the portable charger.

3) I let it sit at room temp for a week and a half. Then I attempted to use the Co2 charger to add more Co2, but accidentally let some pressure out. I was having trouble getting the disconnect on and maybe heard a hiss for about 2 seconds as gas escaped. I then refilled the keg with Co2, exhausting an entire 16 gram cartridge.

4) After two weeks at room temp, I agin filled it with C02 and then placed it in the fridge at 40.

5) after two weeks in the frideg, I tapped a sample. The beer came out mostly foam and settled into a great tasting Oktoberfest. However, afeter the foam cleared into beer, theat beer wasn't very carbonated. There were a few bubbles floating to the top, but not much.

What to do? Did I ruin it by accidently letting out some of the pressure a week or so into it? What options do I have beside force carbonating it (I dont have and cannot afford a Co2 tank at this point!) Can I force carb it with the CO2 Charger?

You are going to need to keep adding co2 as you take beer out or it will come out of solution to fill the headspace. That's why we dispense at 8-15psi using cylinders

You don't know how much pressure you had, so there is even the possibility that it is overcarbed, causing all the CO2 to come out of solution during the pour (making foam) and thus leaving the beer with little remaining CO2 in solution.