Suppressing negative thoughts is a fool’s errand. You can notice them, and react differently to them, but you can’t literally control the thoughts that arise in your consciousness. You’ll work yourself to death trying.
A negative thought is often a right turn on a mental track that leads down a dark hole full of more negative thoughts. You may notice the initial one, but by choosing not to go down the hole, you have in a sense “suppressed” all the negative thoughts that were waiting there.
“In general, people with worse mental health symptoms at the outset of the study improved more after suppression training, but only if they suppressed their fears. This finding directly contradicts the notion that suppression is a maladaptive coping process.
Suppressing negative thoughts did not lead to a ‘rebound’, where a participant recalled these events more vividly. Only one person out of 120 showed higher detail recall for suppressed items post-training, and just six of the 61 participants that suppressed fears reported increased vividness for No-Imagine items post-training, but this was in line with the baseline rate of vividness increases that occurred for events that were not suppressed at all.
“What we found runs counter to the accepted narrative,” said Professor Anderson. “Although more work will be needed to confirm the findings, it seems like it is possible and could even be potentially beneficial to actively suppress our fearful thoughts.””
I do not like the article's use of "suppress" as it is an active and forceful word, that connotes some sort of combat or struggle over the thoughts. It's the struggle that will get you in trouble.
It also depends on the nature of the thoughts and our relationship to them. Sometimes we need to work through negative events, process them, deal with them. This study appeared to focus on future imagined events, which are clearly different, and many of us fall victim to catastrophizing, ruminating, and worrying about things that may never happen, or things over which we have no control.
Practicing centering prayer, or mindfulness meditation, it is a discipline to master our thoughts and not allow them to dominate us. Rather than "suppressing" negative thoughts, they can just be neutrally acknowledged, and released without engaging them. In fact, all manner of thoughts and ideas can be treated this way during a meditation, and we will find that nothing in our mundane lives is so important or devastating that it could take the place of God.
But yes, you can get to a certain age, and with a lack of developed discipline, you will feel trapped and powerless to "control" those thoughts, and the first attempts will feel futile. But it's certainly possible and achievable.