Could you please tell me how you would say these 3 situations

Could you please tell me how you would say these 3 sentences properly.

1 You bring that subject up when it suits you/when it's to your advantage, but when it doesn't suit you, you act like it never happened. Stop using that as an excuse everytime you are looking for one.

- It's whenever it suits her. I'm not to be trusted when it suits her. I'm to be trusted when it suits her.

- When she got something to gain, she tells people I can be trusted, when there is nothing in it for her, she tells people not to trust me.

2 You would be better off not opening your mouth. You sound stupid when you open your mouth/start speak.You make a better first impression if you keep it shut.

3 I keep my computer in the inside pocket of my bag not the outside as I don't want people banging into it. (the bag has to pockets on either side of the luggage with zippers. One faces your leg and one faces outward.) (how would you say this?)

Thank you