When to quit?

[jfh]

I'm relatively new to holdem, fewer than 20,000 hands, both NL and FL. Now I play mostly online Poker Stars at the .25/.50 FL level. I don't win consistently yet and I'm wondering if it is because I don't quit when I'm ahead. Last night I bought in for $10 each at two tables. After about 45 minutes I was $13 ahead. I continued playing for another hour or so and remained $12-$14 ahead. Then the slide started. Playing AK,AQs,AJs,KK,QQ,JJ I lost $15. I reviewed the hands using Poker Trackers replay function, and with the exception of one hand I couldn't identify any mistakes. I just got drawn out on a lot.

When I've increased my buy in by 65%, should I just quit and call it good? I don't like to because I enjoy playing--except when I lose.