To answer it, let's try a little exercise : close your eyes, take a deep and slow breath, and think about an uncomfortable situation you experienced. The first one you can think of. Not a big trauma now, we don't want you to get in distress. No, just something that raises say, a 3-5 out of 10 "bad feeling" (shame, anger, sadness...) 10 being "really bad". How does that feel? 
Now forget about it and CHOOSE to feel good. Can you?
Well done if you truly can. Most people can't.

Let's face it, whether we want it or not, negative feelings and experiences are there. Moreover, the one you just recalled is a conscious one. Your subconsicous holds many more of them. It built a full "database" of these, in an attempt to protecting you from getting in trouble again in similar situations.

Let's try a second exercise : close your eyes, take a deep and slow breath, and think about that same uncomfortable situation, and start tapping on each of the points while describing the story and how you feel, as if you were talking to a very trusted friend, preferably out loud. KEEP AT IT for a few minutes. Progressively start introducing what you'd rather feel about it.
When you feel ready, CHOOSE to feel OK about it. How does that feel? Usually, there some level of relief, or even full relief, If nothing happens, it might be a good idea to explore why it doesn't work.

I see 3 ways to talk about past difficult times or about negative thoughts :

1. Talk about it to try and get it off your chest, or just to share it. It can work.
I have to say, when the intensity is strong, it only brings me some relief, not freedom. 

2. Talk about it to a competent professional.
It normally will help get an understanding of why you're feeling the way you do. It might allow you to let it go. Maybe that suits you, and then, you're sorted :). I am very grateful to the therapists who helped me move forward over years of depression, but I confess I only got full relief and healing after a few months of tapping. Again, that's me, and we are all different.

3. Talk and Tap, or any other form of "energy psychology". EFT is the best one I found so far.
Then you might be getting to another level, I certainly witnessed a lot of people who did.

If you car doesn't start in the morning, you can ignore it, be positive and hope for the best ("it will start, I'm sure it will!").

You could talk about it, maybe it'll help, you might get good tips from friends and fix it ("you know, it was to be expected because of this and that. But now it'll get better!").

Or you can call a mechanic. Tapping is like calling your very own mechanic. You have to explain to your mechanic what's wrong, so he/she can address it. So when tapping, you usually have to state the current situation first (negative feelings, bad memories), before you get to "reframing it" by first considering the possibilities (What if?) and only then, when you're ready, when it does come from your heart, choose what you'd rather have (I choose to let it go, feel safe, forgive etc...).

In Tessa's story, if Anne hadn't accepted to "mull over" a past event, not a nice memory but not something she was thinking about all the time either, she wouldn't have enjoyed this "weight lifted from my chest"...

Does that answer? Any comment, please let me know.