I think one of the best things you can do for yourself is not try to talk yourself out of feeling the way you feel. I mean focusing on the positive is of course a great idea, but I am a big believer in validating your own feelings and hopefully your husband is helping with this.
Having little kids is HARD. It is a blessing and something to be thankful for and it is wonderful, but it is also stressful and a burden and exhausting and can make you feel isolated and incompetent. This is all normal!
When I was pregnant with my second I had these thoughts too. I also worried about my older one (he was 3 when my second was born) and how it would affect him. I couldn't imagine how I'd handle two of them, how I'd be able to meet the older one's needs when dealing with sleep deprivation, on and on.
As much as I or anyone else can tell you that once your'e there you'll be fine, that is not very comforting right now probably. And that's ok. Give yourself permission to feel how you feel. Your body is doing so much right now, your hormones are all over the place, you have a right to experience all of this your own way. You are not overreacting or weird or abnormal. And you WILL be fine once you get there. There will be very hard moments, but there will also be moments where you'll feel like you can climb mount everest with the both of them!
As far as comparing yourself to others you see, that's a slippery slope. You have no idea how those moms feel inside, how much support they may have, etc.
Plus, I think our society sends moms the message that its not ok to express any of the less-than-rosy aspects of motherhood. We're all supposed to act like we're just blessed and in a state of bliss all the time. So we all kind of play that character out in the world, perpetuate those messages, and make it harder on ourselves to experience the whole normal range of emotions because we compare ourselves to others we see in public who are also "in character."
So, in a nutshell, you are totally normal, you are not a wimp, you will be great!!!!!!!!