Deep breaths reeeeka...
It's probably been said before and I know it's hard, but TRY to stay calm. If you look scared or like you have something to hide they WILL stop you and question you.
In saying that, you don't seem to have anything to hide...? I've just read briefly but it doesn't appear that one of your issues is that you have some very large skeletons in your closet....
Just be completely honest and don't offer anything. I don't know why but I was told this too (ie not to have my passport already open to my visa, they would find it themselves)
In saying that, I was almost denied once. The first time that I came here. I said that I was going to meet my boyfriend and she asked where I met him. I said Boston since we had just been chatting online and that's where we had met up physically, it was a neutral place when his friends came to the States for a skiing holiday. She wanted to verify everything I said. First she was going to call him on his mobile, but I didn't know the number off hand so she said forget it (which raised flags to her since I didn't know his number off hand and it was MY BOYFRIEND, she didn't want me to turn my phone on to get the number
) Then she attempted to page him using my name as a reference and she completely butchered it so he didn't know it was me, even though he told me later he heard the page. When she came back, I told her that my name was pronounced THIS way and I think that irritated her. Well she tried again, he answered the page...everything was going fine. She asked him where we met and he said 'On the internet'
She comes back accusing me of trying to hide information, I said that I was not. She told me I was lucky that everything else matched up otherwise I would have been on the next plane home. She let me through. What an ordeal though! The first time was the hardest time gaining entry, and the only time I have ever been questioned about it.
I would think the fact that you've been to the UK before and returned to the US (surely you have stamps in your passport verifying this) would be pretty strong evidence that you will return to the US since you have in the past. They'll flip through your passport and see where you have been and they will see your previous visits and when you returned. If you plan on getting other evidence such as a letter from your boyfriend's mother, etc. I would think this would be more than enough so stop worrying!
I don't know what you should say, only you can really determine that (whatever feels right to you, if it feels WRONG for you to say friend, just say boyfriend)
You will be fine.
edit: just read the link that SillySnip posted - the first story sounds similar to what happened to me only I didn't throw a fit and was allowed into the country!