This really depends on the people involved.

i know a lot of couples that have stayed together though the transition. but i have no idea if it's normal or not.

I was with my gf for 3 months before she found out i was trasngendered. We have been together abot 7 months now.. it wasnt easy at all. Lots of talking to mods on transgendered chats, and just discussing everything. Now im spending over 500 dollars to go visit her and "see" if she can handle me being trans. I even bought a goodfella.. hahaha.

Common? Yes, more times than not for various reasons the relationship dissolves and both parties continue on living their separate lives. That is not to say they don't remain friends on some level though. I only know a handful of couples who have made it through the entire process of self discovery in the journey of a new gender role. Getting past the initial coming out is the easy part. Staying together in a healthy civil relationship for several years is a whole other story though.

I am not saying this to be negative and bring down the happy vibe. It is how I have seen things play out over and over again with those I know and myself as we continue to grow and discover who we really are as people.

It is usually the couples who have been together and have had a relationship for several years or longer before one of them comes out who make things last. If the relationship is strong it only increases the odds but doesn't guarantee anything.

There is so much that goes into being TG that I can't possibly fit all in this little box. When it comes to relationships though, it comes down to who wants to compromise the most to ensure the other person stays happy. Eventually those repressed ideas and feelings come out and that is where the relationships go to hell real quick.

Hopefully this helps.

I met my husband after he had begun living as a man, so I knew he was working on transitioning, but he hadn't started hormones or had any surgeries.
I used to find that he experienced a lot of dysphoria, which put a big strain on our relationship. After we got engaged, I put a big push on helping him start hormones and begin physically transitioning.
It has made a huge difference to our relationship. There is no chance we would have split after that. His moods and confidence improved so much.
We both feel, without my help, his transition would have been harder for him.

I mean, I think in some ways it is common for people to break up. Most relationships don't last forever. In theory, you eventually find one relationship which lasts forever. But before that time, you had relationships that all ended...
As far as changing in relationships, I think people change a lot during transition. Any big change like that in a relationship can often lead to a distance between the two people. I don't think it is transitions fault specifically.