Can you get your girlfriend back after being dumped?

Solution 1:

Curiously enough it comes down to a collectible. Usually when they dump you you cannot date them again, however, if you have collected all the oysters, the 50 of them, that will allow you to date them again:

Collecting all fifty is required for 100% Completion and the rewards for this are $100 for every single oyster found, Carl's lung capacity increases to maximum and his girlfriends react as if he always has full sex appeal.... [snip]
In addition, if a girlfriend dumps you during the game, collecting all fifty oysters will allow you to date her again, whereas normally this is not possible.

They can be re-found in their original spawn locations.