Advice for potty training regression in 3 1/2 year old

I was hoping you could maybe help suggest a new method to help me with my 3 1/2 year old.

Some background: last May she decided that she was done with diapers after months of us planting the seeds in her head of becoming a big girl. She went cold turkey, even at night, and was doing really well. School started in August, very few problems (many more incidents at home than at school or even while out and about). Then, this past January, she started regressing very noticeably. Was having an accident every day. Then, my husband took the kids out of town to visit his family in WY for a weekend and she freaked out - away from home AND away from Mom and absolutely refused to use the bathroom the whole weekend they were gone. After that, she ended up back in diapers full-time for a while except for when she's at school (and she's only had one accident since then while at school. Her issues are at home)

Fast forward to now. After months of trying candy rewards for going, then scolding, time out (briefly - realized it went no where the first day we tried), enticing her with pretty, new big girl underwear, back to rewards, and even taking her to the doctor (she's gotten to the point that she holds it so much that she sometimes is getting compacted and complains about her tummy hurting, even though she's just eaten and that it hurts to go (at the recommendation of the pediatrician, we started giving her Miralax/Benefiber to loosen her up). We realized that a big part of our problem was the power struggle between her and I over her ruining her underwear, so we've gone back to diapers while at home again.

We're kind of at the end of our ideas and wondered if you have anything you suggest to try. I know this is common in toddlers, but thought there might be something that we haven't thought of. She LOVES getting new dresses/clothes so we had been doing this reward system of putting a nickle in a jar every time she went and once it gets to a line (extremely low line), she gets to pick one of 3 outfits I've already bought, and if she has an accident, a nickle comes out of her jar and into Mommy's jar - so the backwards step in addition to the forward step reward, but she's still hit-or-miss getting it done. She keeps talking about the dress she wants to choose when she gets there, so I think I just don't know how to get her motivated.

Help please!

Here's the thing though - she doesn't really act like an out of control toddler. She's always giving me hugs, telling me she loves me, is pretty good with her manners, her favorite thing to do is play songs on her iPod and sing along, or she plays quietly by herself or with her brother with their toys (they get along very well). She rarely has temper tantrums, can be whiny but not excessively so (unless it's a bad day), and when you ask her to do something either says okay and does it or ignores you. Thanks to having an older brother who is also very independent, she picked up a really good vocabulary and assertive attitude early on (she's always surprising people in stores b/c they don't expect such a little thing to launch into a huge conversation with such big sentences and words!).

Like a typical toddler, she is very independent though - wants to pick out her own clothes, wants to buckle herself into the car seat, close the car door, etc so that's why we stopped fighting over the accidents in the underwear - I got hit upside the head by more than one person that I was making things worse by fighting with her over that - she realized she could exert her control here and when I didn't care if she messed up a diaper because it's meant to be thrown away, things did improve a little on her part. But I'm still waiting for them to improve a lot more! Especially since summer's coming up, vacations, swimming, etc and it would be nice to be done with them for that time, for sure. I know she won't be in diapers sooner than I realize, but I wonder if there's something I haven't thought of to try that will speed that process along a little faster - it was really nice not having them for so many months.

So Chickaree, your recommendation is to act like nothing's the matter toward her? What do I do if she starts holding it so badly that she's in pain and we have to have a medical intervention (one of my pilates classmates had this happen with her 4 year old - major compaction - and as it hurt more to go, because it was big and hard, she held it more, like a vicious cycle.) I don't want to let it get that bad. It's very hard for me to step back and let go, but I recognize that that's my problem, not hers. When do I intervene, and how?

Yes, she's still trying to hold it in even when wearing diapers. If I see her "making that face" and ask her if she needs to go, or wants me to help her, she immediately gets defensive and refuses. I guess I'll stop asking her, but we are thinking of re-visiting the pediatrician. She's cried a few times saying it hurt to go - breaks my heart when that happens.

I'm sure when she's a teenager I'll look back on these days and sigh, thinking I was an idiot for thinking this was rough!