How can I stop being insecure in my relationship?

You have to learn to trust your partner. If u love someone, set them free...if they come back they have always been yours. If they don't.. they weren't yours to begin with. If a person has intentions of cheating on you, he will do it at any cost. No level of u stalking him will stop him from doing it.

I have found that figuring out your personal insecurities & focusing on their origin helps to over come them

Well, I think the first thing you can try is thinking about the situation from a slightly different perspective. Maybe try considering the fact that you are in a relationship, which means that the other person loves and cares about you a great deal. With this is mind, feel free to be direct with your partner! You would want the same from them right? Explain your insecurities... You may even find that they feel the same way. A relationship needs honesty and open communication to thrive, so don't be afraid to speak what's on your mind.

Realize that a relationship is a mutual choice: you chose the person you are with for several good reasons and so did they.

This is a good question. To me i think you just have to allow yourself to accept that you're the one your partner chose out of everyone. Another thing just be comfortable with yourself. Find one thing you love about yourself/ appearance and make that your thought for when you start feeling insecure. Also continue to pick things about yourself that you like.

Communicate to my partner about how insecure I feel and try to find ways together to sort this thing out.

Sometimes it helps to open up to your partner about how you feel and he/she will take an action to help you get rid of your insecurities.

Realise that you were a worthwhile person before this relationship started and you'll still be worthwhile if it ends. Your self worth should not rely on others but on yourself as a whole. The minute you realise you are good enough for anyone because you're a good person rather than because you look a certain way you'll stop feeling insecure and realise that they're lucky to have you.

Sit down with that person and chat, for as long as it takes! If things still don't work out, you might need to question your relationship :)

Maybe it is because you do not really know yourself. If you do not know yourself, how can you be comfortable with the person you are in a relationship with?