I worry I let little things bother me too much, especially in my relationship. My partner and I have been together for over a year and are very happy together - however I find myself getting bothered by the littlest things. They lead to arguments and then I'm left thinking me must be sick of me moaning about something silly. For example he had to work, so I was waiting at his house for him to get home around 5 - he text to say he was just going to head to the pub for an hour or so, so he'd see me about half 6. It made me feel so angry. Little things like this get to me, I don't know what to do or how to help the negative thinking go away?

Please work on it a little more. Sounds like some jealousy creeping through also.
Be yourself, and let him be himself. If you keep worrying about stuff like this, he will find someone else. Good luck.

Always think before you act or speak even if you have to count to10 ten times. Its important to practice being cool, calm, collected and in control of yourself in all situations.

It sounds like you also might be setting yourself up for disappointment -- giving him the opportunity to "fail" and then you feel let down (like waiting at his house and then he texted you about his going to the pub).

Stay busy with your own things and projects and hobbies, and connect with each other as an added joy.

It might not all be you being petty. You have to think back to the root of the thing that makes you annoyed. For example: If it makes you annoyed that after work he is going to the pub is it really:
a) You feel Neglected: he hasn't been spending as much quality time with you and the few hours he gets off he chooses to spend at a bar without you.
b) You are Insecure/Jealous/Suspect Cheating: You don't want him there, or there without you because you cannot control or see what he is doing. You doubt yourself or your own self worth and think that he is out looking for another woman.

If it is a) then you could talk to him and ask if you could spend some quality time together so you can feel more fulfilled in your relationship.
If it is b) you can talk to him and both try to figure out the root of that problem. Perhaps you are suspicious of cheating because he flirts heavily with your mutual friends.

I think its all about finding the root of the problem before anything can get fixed. Many times we have an issue, and when we say it or rationalize it in our brains we don't fully articulate the problem. (Example:
Girl: *looks at skanky girl* "I hate that b**tch"
When really, it is: "I hate that b**tch because she is skinny and dressing like that gets the attention of my man. If I had that body though I would flaunt it too, but really, I don't and this makes me mad so I'll just make a broad statement and be done with it."

IDK if this makes sense, but just talk to him about it. If he is a good boyfriend he will hear you out.