So don't think you need to 'break up' to end or change the arrangement. These are by definition no-strings-attached relationships. Either of you has the power to stop or ask for change at any time with no reason or explanation required. Either of you have the power to decline a proposed change as well, knowing that the result could be either no change or a full stop.
You don't need to figure out how to avoid burning a bridge; you are both adults and know (or should know) that the SD/SB thing NEVER lasts forever. Circumstances. logistics, tastes and needs change.
At the same time, SB's are not employees that can be fired or laid off (no pun intended) either. When you have spent a considerable about of time being deeply intimate you do develop a certain level of friendship and affection, and perhaps dependence on each other.
My advice is honesty; tell her it's getting harder to make it work over a distance and you'd like to modify the frequency and/or length of visits.
I did this with a SB in Northern California when I moved south to Los Angeles. She was an Iranian Fashion model in her 20's and we met when she turned 33 and is still super hot. Note that she had previously done the same with me when she was offered a "can't refuse" deal from a high roller that wanted her to move in, finance a full ride to get her MBA,provide a car (BMW) plus a 4-digit monthly allowance. I wished her luck and we parted friends with fond memories.
You can probably guess that the high roller didn't work out and she reconnected with me 8 months later. We didn't skip a beat, just resumed wild BCD about once every 6 weeks. Now that I am over 350 miles away from her, she still looks forward to seeing me when I travel up north. This happened last week. She had a 2.5 hour drive to get to me and she even baked me a chocolate rum cake (non-alcoholic as she's a Muslim) for a tasty after-sex treat. She knows that I probably see other SB's in LA and I know she must be seeing other SD's up north. I don't ask and she doesn't either. I don't even think she needs the money, although she has been very specific that's it's expected when we meet. Her last job before she went to see family in Iran over the holidays was a VP Sales at a company that competes with mine!