As others have said, it depends how significant the problem which caused you to break up is as far as your relationship goes. My ex and I were together for just under half a year, broke up, got back together and lasted a year and a half before breaking up for much the same reason as the last time, only with more finality (at my instigation it has to be said). So it really depends on whether you can resolve the problem and do so properly rather than burying it as my ex and I ultimately tried. Insofar as how to fix the problem of "not being fun to be around"...other than being more upbeat in your demeanour and making an effort to spend time together and enjoy things, I'm not sure what to suggest as that's quite subjective and depends entirely on how both of you are around each other. You can certainly try both of the things I suggested, but if she isn't receptive then that's probably your cue to look elsewhere unfortunately. At the end of the day, both of you need to be on board with things for it to work.
Hope that helps and take care.
