| That depends. If there's a bur or an edge on the saddle, then it's not your fault. Though tuning down to D would only take tension off the string. Maybe that makes you play harder to get the sound you want.
I'd been breaking strings regularly for years (every 1-2 months, sometimes every 3-4 weeks). On my upright too (far less frequently). I eventually came to the conclusion I was playing too hard. I've spent the last several years making a concerted effort to lighten up. Besides breaking fewer strings, I've felt an increase in my dynamic range and agility. I now turn up the amp and relax my fingers some.
I also felt I was losing my style after trying to lighten up too much. So I've compromised. I'm getting the sound I like, I can play the stuff I want to, I'm breaking fewer strings still, and I don't feel physical pain playing 5 nights a week.
After you weigh the health of your arm, I say you just have to go with what works for your music. Oh, and always, always carry spare strings.
Sorry to go on, but:
I've also found that playing flats, I'm breaking fewer strings. I think I play lighter because of their feel, and maybe they're tougher.
I grew up listening to Rush and Yes and thought that they got their sound from whaling on the strings. Years later watching some close-up footage of Geddy I realized he was playing really lightly most of the time. He just drove his SVTs hard. Had I seen that when I started, I feel like would've turned into a different player. | 
