I'm thinking more about how you actually see him, friendly or romantically? If you are afraid of losing him as a friend then it's logical to consider your options, friendships should stay as they are, not get mixed with feelings etc. Your best friend asked you out so he probably feels something more than friendship, sees you in another way. The question is, how do you see him?

I was so terrified of my first big crush that it gave me serious anxiety problems. I barely wanted to go to school because I knew I would see him there. Since you two are best friends, I'll just echo what others have said and tell you that communication is number one, and should come a little easier for you since you know him and know what he's like. Don't be afraid to tell him things!

Hey, congratulations!  Running away from someone and having to break up over it is a pretty big fuck up so the thing you were so afraid of has already happened.  I don't think there's anything worse you can do, other than hurt him on purpose.  And I'm sure you'd never do that to anyone you love so things are pretty okay!  You have done the worst thing, you survived, your friend survived and he still wants to date you.

You'll still worry, of course, but I think you can relax somewhat.

You two might fall out of love in the future.  No relationship is guaranteed to last forever and one or both of you might get bored.  But if neither of you is a selfish jerk to the other you can still be friends once the hurt feelings go away.

Otherwise communicate with one another honestly, don't be selfish or manipulative and enjoy your time together.  A good relationship is also a good friendship.

Thank you guys so much for raising my confidence and making me feel more comfortable about being in a relationship I really appreciate it

Are you sexually attracted to him ? 

Hey there!

Communication is super important! Whatever it may be, it's really important to talk about it with your partner, honesty needs to be a priority in a relationship.

I see that you're scared of messing up somehow since it's your first relationship. What could you do that would be so bad that he wouldn't want to date you anymore? Are you planning on ruining anyone's life in the near future? Are you plotting to take over the world and ban bacon? 

In all seriousness though, I had that same fear when I got into my first relationship. Seriously, you have to do something really bad/terrible for him to break up with you.

And as far as losing your best friend goes, well, at that point it's a choice you've got to make, I think. My fiancee is both the love of my life and my best friend. I think that any relationship you get in, the person is going to sort of be your best friend. What I mean is that they'll become your confident, you can talk about anything with them. You'll be doing tons of things together, hanging out all the time, like you'd do with your best friend, you know what I mean? 

Honestly, I think the best you can do is just be honest with him. You said he's been moving slowly to make you comfortable. You still seem to be uncomfortable, so you could ask him to keep taking things slowly? 

Why don't you tell him that. Communication and honesty is important. I would also keep in mind that being scared of new things will happen with absolutely everything. Sometimes you have to take risks. Trust your gut, use your head, be responsible and you have nothing at all to worry about. 

What're you afraid of?
- has something happened?  (in past or smth)