I want to start by telling you that when I read your question I felt saddened. As mature women, there is a lot of stress over fertility, which only grows as we get older. However, we have more to offer than just our fertility. We can lose sight of that sometimes, because whether it’s society’s suggested timeline or your own dreams you’re trying to follow, not being able to have a child is not an easy thing for a woman to handle. Nonetheless, something has kept you and your boyfriend together for the last three years that had nothing to do with a child. I would suspect that if he’s talking marriage and a child, that he loves YOU, and not just your potential to give him children. So let him love you.
You said your boyfriend wants to get married and have a baby. It seems he has made his choice to stick with you, knowing how old you are. If you haven’t already, I would suggest having an in depth talk about your fears of not being able to give him a child. If he still wants to get married that is his choice to make. Don’t reject yourself on his behalf. It sounds like you would have a child with him, given the opportunity, so you must trust this man on some level. Why not trust him on his choice to be with you.
With that said, I wouldn’t waste any time on a long engagement. Provided that you have a healthy, loving relationship, and this is the man you want to marry, you should plan some type of wedding for the very near future. Additionally, schedule an appointment for a pre-pregnancy check-up right away. Stranger things have happened than a woman in her 40’s having a baby. So I wouldn't give up hope just yet.
Whatever the outcome, you do have options. I just don’t suggest that you opt to deny yourself a relationship with a man who you clearly care about. If it turns out that you can’t have a baby, you can cross that bridge when you come to it. Who knows maybe you’ll never have to journey over that bridge; and if you do, you will find there is more than one bridge to take. At least you can cross it holding the hand of the man you love.