It sounds like the main issue is about communication. As young children we learn how to talk long before we are even able to remember learning how to talk. We think of communication as entirely natural, not as a skill we need to learn or polish. Some people are natural communicators, but many people feel stuck and ineffective just like you do. There are several reasons why this happens to people.
Shyness and lack of self-confidence are certainly the main culprits for most people in these situations. This factor can be made worse if the situation is one in which you want to express disagreement with the opinions of other people in the group. Expressing a difference of opinion can lead to conflict with others and most people want to avoid conflict at almost any cost.
Conflict is tricky and can lead to hard feelings and disrupted friendships. It may seem like it isn’t worth taking the risk. If this is part of the problem, it is important to learn how to handle conflict in a self-caring and tactful way. There are books, classes, and counselors who can help you learn this skill. Learning how to manage and resolve conflict will give you more confidence and make you an asset in any group situation.
The other issues you talk about seem to be about losing your focus when listening to other people talk and your worry that others are not hearing what you have to say. Getting sidetracked by what others are saying can also be called “actually listening” and is a strength, not a weakness. Still, you might have a brief written or digital note to yourself that you can refer to that helps you come back to the points you want to make after listening to other people’s points of view.
Finally, keep in mind that you can only control how you deliver the message, not whether other people hear it. It is also possible that they are hearing you, but disagree, or just are quiet types who don’t actively express their feelings about what you are saying, even if they agree with you. In other words, they might be listening and it could be your perception that is not correct. Either way, consider taking a communication class or reading the old standard guide to successful communication “How to Win Friends and Influence People” by Dale Carnegie. If you are a little braver, join your local Toastmasters group and get lots of experience speaking your mind in front of others.