It may just be that you're not spending long enough on foreplay. My boyfriend and I often have the same problem, but it works out well for both of us because we spend a lot more time "revving the engine", so to speak. If that still doesn't work, there's a reason lubricant was created. There are so many different kinds as well. You could try taking your boyfriend shopping for some. You can pick one that you both like, plus, shopping for sex toys and accessories tends to get a bit silly, which is a great way to get you both laughing and feeling more comfortable about the situation.
Have you tried oral before penetration? Saliva is a natural lube, and an enjoyable one at that! Failing that, there are many "trendy" lubes available, with flavors and/or effects that simply add to the fun, playful nature of sex. It's nothing to be ashamed of. Sometimes hormonal changes affect our personal lubrication, and they could be down to medications, weather, food, mood, etc... the list goes on. Surely it's better to have to use a little lube and get to have intercourse with your partner, than not use any and have to stop because of the pain? From a personal point of view it doesn't make a difference whether or not the man is circumsized.