For an INFP, directness in confrontation is a critical element to maturity and success in relationships. But before an INFP develops it, there are a lot of defense mechanisms built in to protect from guilt, from being hurt (distrust), and from facing accountability. Also sometimes when an INFP is in a situation where the other person is at fault and they are afraid to confront them, the INFP will instead place the blame and guilt upon themselves.
Two critical components that you might see there are "Trust" and Self-Value" (in varying degrees depending on the individual). As the partner of this INFP, trust and self-value are the areas you want to help bolster for the INFP to aid her in loosening the power of her unhealthy-type emotions and the defense mechanisms that correspond to them. This requires a combination of gentleness and tough love. An INFP will not mature when only coddled, nor will the INFP mature when only given toughness.
When you are confronting the INFP about a pre-existing issue or something in the moment, it is important for the INFP to know that the conversation and environment will be emotionally safe, so establishing that is paramount, otherwise the internal thought might be, "Oh great, I know where this is going," and defense mechanisms rise. Therefore, before you deliver your thoughts on an issue, tell her in whatever way appropriately matches the issue, that you would like to hear her feelings on the situation. You may encounter an "Are you trying to say that you think I..." response, but even if that happens, you can still establish her trust. You can say, "I have concerns about this situation, but I want to hear how you feel about it, too."
One of two things are likely to happen. Either the defense mechanisms that have risen up will stay engaged due to pride or lack of trust, or she will relax and appreciatively give you her feedback. The latter is preferable, but the former is okay, too. If she is still defensive, that is when you become firm. You tell her that you would like to discuss this, but cannot continue to do so unless she is willing to do so reasonably. What this whole approach accomplishes is to tell her firstly, that you are accepting of her view and emotions, BUT, that she cannot manipulate or make any progress through her defenses. It may take many episodes before this becomes consciously recognized and accepted by her, so it may require you to be patient.
One other thing is that you need to have her understand that if she is successful in delaying a confrontation because of her emotional state, she can't avoid it. You can tell her you plan to talk about it with her later. Avoiding is a tactic many INFPs use, and we are very good at it.
As for yourself, though...use your best judgment to ascertain whether an issue is really critical to bring up or not. You may view any violation or smudge in the relationship as something to confront, but by doing so continually, it may only damage your INFP girlfriend's trust, because she may feel like you see too many problems, and then see herself as the problem. That is why when you ask her for her views and feelings, you shouldn't do so to patronize her, but really hear her out, and you might detect that the situation is different than you had originally seen it.
If her defensiveness and/or avoidance is persistent over time and she seems unwillingness to meet you halfway and exhibit trust, then she may not be ready for a relationship.
Another side note, the "shell" you describe is often a place where manipulation is carried out by INFPs. It is also a place where there is hurt, and understanding and affirmation is needed, but utmost caution should be exercised. It is a self-protection mode, and therefore, whatever self-protective result that is desired, will often be carried out through a form of manipulation...manipulation to find validation, manipulation to avoid, or to get sympathy. So, though validation and affection and sympathy is important, take care that you aren't delivering it in reponse to a manipulation.