Why would I be slow to respond during serious conversation?

My girlfriend and I have been dealing with some very serious topics lately and she is getting upset because she feels like I am not responding to all of her feelings and concerns. I have tried to explain to her that I need to slow down the conversation, when she talks she says a few things, pauses for a minutes, then continues with more , pauses for a minute and continues, until she gets mad because I havent addressed anything. For me, when she is talking I am listening, and trying to come up with the best response, but I dont have time to come up with a response before she continues talking.

She feels like my non-responsiveness is a lack of willingness to talk or that I am not listening. She also feels that I take so long to respond because I am "filtering" my thoughts and feelings before I respond, and that if I were answering immediately that I would be giving her my true feelings, not something planned out.

Last night for example, we dropped off her kids at camp, and on the 1.5 hour drive home she started expressing to me that she is not feeling special, and that incidents in our past are causing her to wonder about how special she is to me. I was trying to come up with responses to her comments, but it was taking longer than usual, partly because I was also driving.... When we got home she was understandably upset. I asked her if we could try something different. I sat down and wrote out what I was thinking and when I tried to show it to her she got mad because I wasnt verbally expressing it to her.

I am not the type of person who is used to dealing with and expressing my emotions. I have found someone who makes me want to change that and we are at a place where communication is extremely important. I want more than anything to be able to give her what she needs (i AM willing), but when we are talking, I just cant blurt out things like she wants.

I think part of my problem is not being accustomed to sharing like this, but I feel like there is something else going on with me. I think about what she is expressing, I process it, but a lot of times I just dont have an immediate response. Sometimes I feel ADD, where I cant focus 100% on what my response should be, instead i flash through a dozen different things, from things I need to do around the house, at work, then back to the conversation, then "hey look at the moon", etc.