What do you mean my "collects hard"? Does she tuck into a ball and suck behind the bit?
I would think with any OTTB, retraining to accept contact is kind of a backwards thing. You would first have to teach her how to carry herself with little to no contact to get that whole concept of grabbing the bit and run out of her head. For now, forget about contact and collecting, and just get her used to rhythm and stretching out her neck. Walk and trot around on large circles or take her out hacking and let her find her natural rhythm so she can start relaxing and reaching forward with her neck. Most TBs are very quick learners, so depending on how much baggage she's brought along with her, this will take from a few rides to a few weeks or months. The initial goal is just to get her to relax.
Once she's got the idea of stretching her neck out and letting go of the tension, start taking up light contact. The big caveat here is, be sure to follow her motion with your hands/elbows. At no point do you want her to feel she hit a wall. You should be able to feel her mouth on the other end, but she should not feel like she is being hit in the mouth. Over-exaggerate the motion at first until you know you are in sync with her.
Once she's starting to accept your lighter contact and not tucking her face to her chest every time she feels the bit, you can start to take up some slack, but again you need to keep following her mouth. I don't know about other breeds, but a TB will happily reach into soft allowing contact but slam on the brakes when they feel the wall on the other end. They are night and day that way. Also remember to reward her when she does what you are asking. Any time you feel her try to reach for the contact, soften you elbows and tell her what a superstar she is. Any time she reverts and sucks back, ride her forward on a large circle until she figures out that is not the solution. She'll get it eventually.
As for the bit, the mildest bit I know of is a french link snaffle. The rings are a more personal choice. O rings move around in the mouth the most, egg butts to a lesser degree and D-rings offer the least amount of movement but can encourage a locked jaw. Maybe split the difference and try the egg butt first.
Hope this helps.
You just have to see your distance...you don't have to like it.