It's been almost impossible for me to orgasm since starting Zoloft. Any advice?

I'm a male in my early 20's and I take 50mg of Zoloft a day, if that is relevant.

I started taking Zoloft a few months ago and experienced no sexual side effects. Then, because of the incompetence of my pharmacy and/or doctors office, I couldn't get a refill and had to stop cold turkey for a few weeks. I've recently started taking it again, but this time it's really messing with my sex drive. I still have the mental desire for sex, and I have no problems getting erect, but it's almost impossible for me to actually finish. I usually masturbate at least once a day, but now, even though I'm still horny, it just doesn't seem worth the effort. When I'm with a girl, I'll stay hard the whole time, but even if we go at it for over an hour I can't even get close to finishing.

Does anyone have any advice for how to deal with this? I've heard taking welbutrin can help, but I don't really want to do that because I've heard it does not mix well with alcohol at all. Is there anything else I can do? I'm thinking about just stopping taking the Zoloft because I honestly haven't really noticed any effects from it, positive or negative, aside from these sexual side effects.