It's all going to be okay.
It is, really. But also, be prepared that this process could be extremely slow. Maybe you will be one of the lucky ones whose ms sells right away. But for many (most?) of us, the process can drag on, and yes, it's possible it won't sell. BUT you're working on your 2nd book, which is huge, and great, and as much as possible (I know it's hard), put your focus there. Because there's a good chance that the book that landed you your agent will not be the one that gets published first. I know you don't want to hear that. This is your baby, and you want the world to love it. It eats me up inside that my manuscript is just sitting there not getting published, getting staler by the day. (It's contemporary YA and it won't be long before it sounds dated.)
My ms went on sub in March, and it was an emotional rollercoaster. I felt the highest highs and the lowest lows of my life. Getting rejected has gotten a little easier every time, but it never feels good. I will say that no editor has ripped my ms apart, or if they have, my agent has shielded me from that. I usually get vague feedback that I can't really work with. Occasionally the feedback makes me question whether they got past the first chapter. I've also gotten a lot of really nice feedback about my writing that I try to remember, instead of just focusing on the "I got rejected" part.
I've considered many times giving up, except it's never really been an option. What made me feel better was realizing that I'll continue to write no matter what happens.
My agent just sent my slightly revised ms out to a new batch of editors on Tuesday. I'm much calmer this time around. I've adjusted my expectations--I no longer dream of a bidding war. Now I dream that someone, some day, will publish it, and I'm focusing on my second ms.
I don't think that ended up sounding very positive at all. But I also think it's important to have somewhat realistic expectations. No matter what happens, you are a writer, and you will keep writing. One thing that made me feel better is something my coworker said about getting published: "It's like water. It has to find its level. Some water takes longer to get there. You can't rush it."