> I guess my question is Am I doing anything wrong, and if so, how do I fix it?

It is not that you are necessarily doing anything wrong, although with some insight and knowledge, you will be able to become better at what you do.

> someone posted something saying that you shouldnt finger a girl for a half hour and that most girls get bored with intercourse after 10 min or so.

I believe some clarification is required. There is a difference in fingering a person's genitals for half an hour and making out in general for that or longer. What do you classify as "fingering"? Do you mean the clitoris? The vagina? Both? Her genitalia as a whole?

A couple certainly can and usually do make love (meaning making out in all of its various forms) for longer than half an hour, often going an hour or two or more, depending upon time. inclination, and, mood. Making out is key to making love. It seems to be a common misconception particularly among young guys that the way to an orgasm is through lots and lots and lots and lots and lots of stroking. WRONG. Stroking maintains a preexisting high level of arousal; thrusting peaks this and triggers a climax. Intercourse works best when it does not begin until the man has become very highly aroused and is on the brink of a climax yet can still maintain control. We achieve this high state of arousal, best and ideally, by making out--Necking, Petting, progressing to Heavy Petting, and eventually moving on to Foreplay--all of these steps being inclusive. This means that we move from one step to the next within each category and then revisit each of the previous caresses as we add the next caress, and so on and on.

> At the same time, it seems to take me at least a half hour to get her to orgasm, no matter what I was doing.

Making out is more effective the longer we do it within reason. It is safe to say that women particularly require at least a half hour in order to get all reved up because of the way they are "wired". Of course, all this reving benefits the guy as well even though we can be good to go within minutes. Just because we can be does not mean that women can always be ready and this is a critical lesson that seems to have escaped many a young fella's sexual education.

Now that the two of you are on the virge of orgasms, intercourse can begin. What I've said in the past is that if intercourse lasts much longer than ~ten minutes, most women tend to become bored, sore, and tired. This being the case, work within this window. After climaxing, if the two of you desire, you can take a break for a few minutes and then enjoy one or more orgasms. Women can experience multiple orgasms within moments of the previous, and, have many more than guys. A guy's refractory period (down time) is usually between ten and thirty minutes depending upon age, drive, and how he's "wired". A couple will often use his down time to let her enjoy more orgasms while keeping his level or arousal fairly high.

> I could cum within the first minute or so if I wanted to, but I dont and i end up taking a long time, like 45 min minimum, and thats with me really trying to get there.

Are you making out during this time or humping? See, above.

> My gf always complains that either she is sore after sex, or if she is stimulating me in other ways that her arm is "going to fall off" or her jaw is really sore.

Asked and answered. Put more time and effort into making out, and do not rely upon extended Foreplay to build your respective levels of arousal. The same holds true for stroking.

There are two reasons why her arm probably feels like it is going to fal off. The first is that she has not used her arm and wrist very much and has yet to build up strength using them like you would when masturbating. Second, is that she is probably spending way too much time trying to get you to the point of your climax and not doing it in a way that you have come to rely upon over the years. Please understand that right out of puberty a boy will begin to put together a routine of movements, rhythms, and pressures, that he then uses day in and day out to bring about an orgasm. If we vary the way we do it too much either an orgasm will fail to materialize or it will be less than expected. The fix is for each of you to learn how to duplicated each others method of masturbating.

Here are some informative Sticky posts for the two of you to read and apply:
How Do I Get Him/Her to Orgasm From a Hand/Blow Job?

For Women Only- Help! Why Can't He Make Me Orgasm?

I hope this is of help. Got questions?