What is the best bit for my older horse(s)

I am 44 years old & just acquired two horses. One is 17 the other 12, both mares. Both are supposed to be good riding & gentle. The previous owner used a D ring. However...

I am new to horses to a certain extent. I have been regularly given horses to ride, but never *tacked* up one. The D rings did not work for me - I lost control of one (17 year old) & the other I (12 year old) stopped, dismounted before that happened. (if it matters, both horses have been throughly checked by the vet & farrier and given an OK)

I have been told I need a curb strap with older horses. Also I would appreciate in suggestion of sites dealing with the mechanics/operation of bits (I'm obviously NOT using the right keywords). I've had the horses now for 4 weeks - each ridden once. Now they are settled in, I'm ready to move forward. I'm sure I'm forgetting a lot, so ANY advice is also appreciated.

The owner sounds like he or she may have been more used to the horses and more advanced-- I suggest you move up to a bit with a curb chain and maybe try a tom thumb. The only thing about tom thumbs is that you can't use ALL hand all the time-- you need to use you posture aids as well, like legs, seat etc... If you use too much hand with a tom thumb it can be a very harsh bit and pretty soon it will stop working and your horse could develop a 'hard mouth' After you get to be a better rider and get more used to the horses and vice versa you should go back to a D ring-- i love those bits =) Remeber, don't expect everything to work out RIGHTAWAY-- be patient!