I am a female who is with a great guy. My guy always has a hard-on during oral and foreplay but goes limp during intercourse. This makes me feel bad and think that it has something to do with me. I cannot bring myself to talk to him about this even though it bothers me greatly. How do I approach this great guy without hurting his ego?

Very carefully.

First does he know that this is a problem? Has he had his little friend checked out by a doctor? How old are the both of you? Is this a consistent problem? IE, out of the last 10 Sexual encounters how many did he go limp for? How does he masturbate? What sort of medication is he on? What type of BirthControl are you using?

There are a few things we need to cross off the list or concentrate on:
1). Medical issue. There are a few medical issues that might be at play here causing him to go soft. General health, weight, or substance abuse coupled with physiological issues.
2). Psychological issues. There could be a mental block that he is not aware of. For example if you're using a less then good contraceptive method, withdrawal for example, and he doesn't want kids, or you've had a pregnancy scare. He could, through no fault of his own, be protecting himself by not actually penetrating.
3). Masturbation. Depending on how he pounds his poodle, he could not be getting the stimulus he needs to stay hard. Depending on what is going on around his penis the sensations can be quite different. Vaginal, oral, hand, anal, armpit, fleshlight, and microwaved watermelon can have very different feelings. Some better then others. If he is used to the death grip then the silky sensation of vaginal might be too little. This is a recoverable problem.

Everything can be solved here. The thing is that, as you noted, it is going to require communication. There is bad communication, i.e. signing him up for all the Viagra spam lists, and good communication. This is where you talk to him about how you're feeling and see if there is anything that can be done. The key here is to not deflate the ego. Approach it as a concern for his well being, as problems with the little monster can be an early sign for other issues.

The last thing is, this isn't you. This is almost all him.