A big decision
Having struggled to lose weight in the past, it's natural that you would have serious concerns about it in this situation. But consider how different a person you are now than you were when you had your now 8-year-old. You're older and more mature. I'm betting that you're also better informed at this point about food choices and fitness. I had an easier time losing the weight I gained with my 4th pregnancy at age 29 than I did with my 1st at age 21. I exercised during my pregnancy, ate better, and returned to exercise more quickly and sensibly in the 4th than in the first. Yes, you will gain weight. That is normal and healthy, and you need to see that as a good thing for both you and the baby-to-be. But that weight WILL come off if you use the resources you have available to you. This forum would be a great place to get support both during your pregnancy and post-partum. Doctors are much more supportive of maternal exercise than they were 8 years ago. You've shown that you have the will and determination to lose weight. You can do it again if this is the choice you make.
That said, please spend some time making sure that all of the anxiety you're feeling is really about the weight gain that accompanies pregnancy. It may be that you're focusing on the concern that is closest to the surface,and missing something(s) deeper down. Are there unresolved issues surrounding your son's birth ( traumatic labor and delivery, for example )that may be adding to your fears? Are you worried about how your partner will see you when you're "large with child?"
I'm not saying that you are, but sometimes it's easier to focus on the obvious.
Babies are wonderful gifts, and if you choose to add another child to your family we will be here to support you in any way that we can. But it's a lot easier to deal with emotional issues before you're on the pregnancy hormone roller coaster.
You're very wise to be asking these questions before you're on the ride of your life!
Good luck!
Erin