Hi! I was scared of integration but felt that it was the best thing for me to do. I chose my first integration.
What happened to me was that a part introduced herself to me, and then told me things that
I'd been hiding from myself- forgotten- and that I didn't need them anymore.
I cried, and cried, and it was painful, but I needed it. And when it was over, suddenly everything that I was confused about made sense- I could see the world from all their points of view, and it made up a big picture that made things seem less overwhelming and scary than they actually were.
I had all their thoughts, and abilities... I'll mention that the scary thoughts were less scary because I could see from other points of view so I could see they were irrational

I still have my gatekeeper, F. We still grow together. I don't know if he can actually integrate, he doesn't seem like a part. We have full co-consciousness now and we work together.
All my other parts/alters are us now and we can be all the silly and crazy we want to instead of them holding the personality, we have all of it!
The "real" challenge comes after integration, something no one really warned us about.
Then comes learning to live life as a "singleton" and to cope with everyday life without switching.
It was a little lonely at first and I wished a lot to be able to switch or just fade out of bad situations again, but I couldn't. But over time I got better and better at dealing with everyday life without dissociation. I learned new ways of doing things and coping with bad situations.
It wasn't easy but it's nice not having things so hectic anymore.
The second integration came about a year after the first, I had two previous parts fused together that surfaced as time went on, and they/she spontaneously integrated when I discovered that I had rejected the "dark" inside me and accepted that I was human and could have good and bad thoughts.
I didn't even notice until I realized that I was thinking the same things she was.

I've also had integration happen too soon, early on in my therapy. I had an integration happen- I forced it and it was bad. It only lasted an hour or so, because I wasn't ready for it.
I've become more of a "whole" person.. I used to be so stiff and joyless/emotionless. Now I have a full range of personality and emotions that continue to grow where I left off
Good luck in your healing!