Foreplay isn't so much for "loosening up" the vagina as it is for lubricating it and getting each other more excited about going at it.
Everyone worries a lot about their first time, but it'll all be okay. You care about your boyfriend, and if he only lasts 30 seconds, well than so be it. All that means is that he's really turned on, and that's NOT going to be a turn off for you. It all comes from experience.
Talking about things first is good, but it can also make you overly critical of yourself and worry more about things, like seeing each other naked. Why don't you fill up a nice, warm bubble bath and turn all the lights out except for a candle a few feet away from the tub. Then bring your boy in there, shut the door, and start playfully undressing him. He probably won't worry so much about what he looks like with the lights out and will be so excited about you wanting him naked. Just enjoy each other in the bath and don't move too fast. If you have a feeling he's self conscious about a certain part of his body (maybe he's packed on extra pounds in his stomach or really hates his skinny legs) try not to dwindle there so he doesn't feel the need to be embarrassed. If the baths hot, you probably won't be able to stay in there more than 10 minutes, and this could be a great way to mention "taking it to the bedroom" for the first time. Leave the lights out, and don't worry about LOOKING at each other so much as enjoying it. It might not be as perfect as a movie the first time, and that's completely okay! You'd rather learn together than have him come to you and having his "routine" down just as well as he has down tying his shoes, you know? Especially not for your first time. Don't worry about the pain issue. Yes, it may not feel like heaven the first time, especially if you're not 100% confident with your body or sure of yourself, but it's nothing to dread. If "getting it in" is hard at first, just roll him over, climb on top of him, and spend some time kissing and just "feeling sexy" to get you more in the mood and try again. It's not going to feel unbearable, and I'm sure he'll be gentle. And if you do end up with endometriosis, I'm sure it may feel more uncomfortable, but you'll be lost in the moment and wanting to take this next step so bad that it'll be bearable.
Hope I have helped!

Just try and relax.