Despite all the "evidence" that you can do it....two things still remain:
1. You detect FEAR (don't ignore that!)
2. You're still NOT married. If you want to keep the stress level down and set a great example for BOTH kids...do what's right and get married FIRST. Give yourself some time to adjust to married life, etc. If he has a year or so to settle into married life and day in & day out fatherhood, the fear will subside. Fear and pushing people into things they're not ready or mature enough for can COMPLETELY sabotage a relationship! ARe you willing to risk that? Adding another child now will only complicate things financially - no matter how well you're doing you have no idea about any complications, etc that may arise) as well as time AWAY from him and your daughter. When people get married there needs to be concentrated time on the two of them. With a ready made family, that's hard enough. Personally, I think you'd be making a HUGE mistake! If he has fears now, what do you think adding another child will do to change that? You'll have LESS time for him if you add another child to the situation. Your daughter deserves to have time to adjust to being a family too. She'll be dividing time with two more people, not just your husband. REGARDLESS of the fact you want your kids close together, it may be BEST that they are a little further apart.
If you wait, you'll have a child that is certainly more independent and helpful and hopefully a great role model. At that age.....they WANT to be helpful and looked up to. I know Abbie does. She's 6. She LOVES helping with the little ones at church. In addition, your husband will hae a chance to put more focus & time on raising a child from birth. As you already know, infants and toddlers take and INCREDIBILE amount of time. Every SECOND is worth it, but they needs and DESERVE the time & attention.
Might want to take some time to talk about it from BOTH perspectives with your future husband. Don't let emotions take the upper hand. They can destroy relationships.....whether that be jealousy, insecurity, fear or whatever. That's almost always a mistake!
Make your decisions based on what's best for everyone! Look at it from BOTH sides. You can even make a list and categorize things: finances, alone time, mom & dad alone time, chores, emotions (in each situation) etc. and TALK ABOUT THEM. Make SURE you listen and honor your future husbands wishes. Some women do what they want despite what a husband or boyfriend wants. It ususally ends up destroying trust and respect and many times destroys the relationship.