It’s been about 6 months since I haven’t spoken to my ex. I miss her. I’m thinking about writing her a letter to get her back or maybe visit?

i would contact her if thats what you want to do

Visiting her would be a much better idea than a letter.

Telephone call or letter may be preferable, she might not want you turning up on her doorstep,  there was a reason you broke up.

One cannot walk back into one's life at whims and fancy?

Relationships are habituating---sometimes they take a while to wind down.

Talk to her and see how she feels about what you want..

Thing is, all love has their issues. You broke up for a reason. With such little information, it's a little hard to give you a good response. Some girls want their ex's back. Some don't want to have anything to do with them. It all depends on how you broke up. Think about it. What did you do to break you two up? Or what did she do to break you two up? Without knowing that question, you might not find an answer.