What if it's a rebound? Perhaps it is, only time will tell. Most of us who've had more than one relationship have had rebounds. If it is, you'll get over it and move on, with a bit more life experience to draw on. (Hopefully largely positive!)
What if my bar is so low and my self esteem so shot to bits, that I'll still accept bad behaviour because that's all I'm worth? I know this isn't true but it's ingrained in me. New partner isn't perfect but neither am I. I can't hold him up to a standard of perfection that is unrealistic.
In what way is your new partner "not perfect"?
What if because it's so intense now, it is doomed to fizzle out? Again, only time will tell.
What if I ruin it? You won't. You may, however, find that the relationship doesn't work for any one of many reasons, and you may frame that in your mind as if you ruined it. Please try to recognise this kind of thought pattern. If it doesn't work out, that's not the end of the world.
What if he sees the real me underneath and it's ugly? The real you is not ugly. If he sees the "real you" and doesn't like you, then you're not compatible, that's all. You can't be a cardboard cut out fake, it won't make anyone happy. You have to be the real you, warts and all.
Be mindful of recognising if you're accepting bad behaviour but please try not to worry so much about the relationship not being the right one for you. If it turns out it isn't, that's not the end of the world. Enjoy it while it's here - if he's treating you well, of course, and let time show you where it'll go.