Hello everyone. Before I start I'd like to say that this is a pretty nifty site they've got here!
Anyways...
My girlfriend and I have sex. A lot. And each time when we're done she says she felt great, she's tired, and she can't go anymore. Only problem is: she hasn't orgasmed. She never has - excluding clitoral stimulation - and said that she, quote, "Feels the cum in there, but can't let it out." And when we're done she feels like she has to pee really bad, but then says it hurts like a bitch. What the hell? Again, she always says it was great - which is a good thing 
- but she can't let it out after it builds up. This gets to the point where she can't even go anymore sometimes, and I have to either stop or force mine out. I doubt penis size has anything to do with it, and when I stimulate her clitoris while we're doing it, she said it makes it harder. Any girls out there wanna help a desperate man out, it'd be much appreciated. Just tell me what to tell her to do!! Please 
Both of you need some basic information on female sexual response. Some of your question indicates that neither of you really understands what should be happening. Get a good book (Our Bodies, Our Selves) and learn female anatomy and what does and does not contribute to a woman's arousal.
A reality is that about one woman in four never experiences orgasm and about half do not reach orgasm through penile thrusting but require additional oral or digital stimulation. All that messing around leads to the feeling that one has to pee. The best "training" for a woman is masturbation. The best thing for each of you is to delevelop realistic expectations.
I have had a girlfriend with the same pee thing too. I have found that extending the amount of time durring foreplay has helped. This gets her juices flowing, and RELAXES her.
You could try slow gentle clitoral stim, though don't over stimulating the area. It's a clit, not a pebble to polish.
Also....the best "Big O's" my girlfriend has had, she said were when we weren't trying to, and we just got caught up in the moment.
Don't worry so much about her climaxing, a watched pot dosen't boil any faster. Take your time, relax, and let both of you enjoy the love making....soon she'll be cumming so hard you won't know what to do.
There are lots of women that only can truly have an orgasm by clitoral stimulation. I am pretty much one of them. That is why I often use a vibrator durning intercourse on my clitoris to achieve great orgasms.
Not to sure about the peeing thing. Make sure she goes before you start might be helpful. I know if I have to pee and contine with intercourse the pain does build up.
You could be hitting her G-spot which gives the feeling that one has to pee.
There are really good things in this forum about female orgasms. I even posted a good article on it. I suggest you do some reading from what the other women have to say on it.