I know what a hard decision you are facing, and I think you should trust your gut. Try going to sleep 1 night thinking "I am NOT going to do this" and the next night, think "I am going to do this" and see how you feel each morning. You will know what is right for you.
I have to say that the answers from the moms who "do not believe in IVF" really made me angry. Clearly, they have never dealt with the pain of infertility. We live in an amazing time, when medicine can help us do many things that weren't previously possible. But the decision to conceive, either through IVF or naturally, is a hard one. And the impact of another child in your life is big no matter how you got pregnant. If you want another child and IVF is the way to get there, go for it and don't look back. The hormones, the shots, the doctors appointments... they are all hard, but that's not what you'll remember when you look at your beautiful children.
After 6 miscarriages, I had a child (a single birth) through IVF, and I was lucky enough to have leftover embryos. We were all set to do another transfer and try again when my husband lost his job, and we put the process on hold for a while. In reality, I was having the same kinds of concerns as you. I was so all-consumed with my love for my first child, didn't know if I could find room in my heart, my daily schedule, my house, my bank account for another child. But when we put the process on hold, I couldn't give up the idea of trying again.
We are 8 weeks away from baby #2. And I'm still nervous about how I will be able to mother another child. But I know we'll figure it out. And I know our family will be richer for it. I know if will have a big impact on my first child, but I also know that having siblings was a joy for me. You already have 2 kids; you already know how to divide your attention and love them both.
There are no wrong decisions here.