I had to travel for work for about 5 years. My experience is: Don't discuss your mental illness issues with coworkers Even if she is friendly now, if something happens, she may spread your story around in an unfriendly manner, giving you a more difficult situation. Whenever I talked to coworkers I always regretted it. They were no better equipped to help me than before they knew because they are not professionals nor do they have similar issues. People judge mental illness -- right or wrong. Whenever I told coworkers, ultimately, they treated me differently -- with less respect and feeling okay about "taking my inventory" (i.e. telling me what to do, giving me advice, telling me what's wrong with me.)
Instead, make a plan using support systems that are more reliable. Set it up to check in every day with someone at home that you trust -- even your therapist, if possible. Bring along things that you know make you feel good and are distracting. (Music, bath beads, a good book). While you're in the car, feel free to read or put on head sets and listen to music. You're not required to entertain. If you have a gameboy or something like that, have at it. A laptop computer can also be a good distraction. You're also allowed to sleep.
Don't let the conversation turn to discussing your emotional mind state. Tell
her you'd prefer not to discuss it. There's plenty of other topics -- office gossip, politics, the state of the world, the war, 9/11, the latest fashions, the latest music, cute guys, popular movies, favorite books, etc. etc. There is a tendency for people with BPD, especially in combination with PTSD, to think they have to disclose their pasts and conditions. NOT TRUE. Keep it professional with coworkers and you'll benefit in the long term.