By staying a full six months you have established you have no strong ties back to your 'home' country.
By not working while in UK you have established you were unemployed, so a risk for looking for income (illegally) while in UK.
By overstaying you have established you cannot be relied upon to leave the country of your own accord when required to do so.
By being with your fiancé, who presumably has the right to reside in the UK as you have not mentioned he too overstayed, you have given reason why you may become an illegal immigrant if allowed to re-enter.
By staying a full six months and more in UK you have somewhat exhausted plausible reasons (such as tourism) for a return after a short interval.
It is very likely that the length of your stay will be noticed and that you have overstayed. It is very unlikely that you will be allowed back in before expiry of your current visa and not likely you will be granted a further visa after a short interval.
If you do attempt to return under your current visa you will probably be refused entry and make it even more difficult for you to obtain a further visa for UK, or elsewhere.