Romans,
As a dil , I was always quiet , shy (in the begging ) and uncomfortable opening up because of my past and fear of judgement (as she is VERY judgmental) . My DH mom was and IS very uptight . In reality I'm loud and talkative and laughing . After we had our first official falling out , it has never been the same and I would distance myself even more . BUT I would still converse about general things or unless its about her , and not me ( I can not talk about myself without her ignoring me or changing the subject EVERY TIME literally) I still smile and nod but if she said or did something disrespectful to me I would sometimes talk back in a serious tone and distance myself again.
SO, this is why its hard for a dil to open up. If she is keeping her distance , ignoring you, or being standoffish ... you might be saying or doing things she finds insulting.
If that is not the case then she might truly have an anxiety/depression problem that she really needs to get help for.
or she could still be holding a grudge from your past mistakes and nothing you do will ever make it up to her ,ESPECIALLY if she feels you are "still" meddling or saying hurtful comments to in some way. If you have done all you can to make it right, and still nothing then she is immature and hopefully( and I think she will ) realize and do things differently for the sake of family

.
I'm just saying, this from my perspective and experience as a dil. You sound like you truly want things to change between you two , obviously there is a big elephant in the room and maybe talking to your son and asking what the problem she has with you is and why she avoids you . She might get offended first that you went to her DH and not her (although understandable ) and might feel like you are starting a problem with her but it CAN give her the opportunity to open up since you are wondering about the problem and not bashing on her to your DS.
Wish you all the best!!!
I know there ARE really true MIL's who are good and do want a close relationship with their DIL's just like the true DIL's out there who do want a real relationship just as much.