what I feel like saying sounds pointless sometimes so I don't say it.
I know the feeling. I am like that myself. At times, it can seem like things are unnecessary to be said and it can be an internal struggle to make yourself say it regardless.
I have had a number of interviews lately and have Alhamdulilah seen quite a few positive responses.
It is all about how you sell yourself.
You need to
sound important (while not overconfident and arrogant). Whatever you say, you need to expand on it (even if it feels like you are chatting rubbish, just do it). Make everything you say sound interesting and important and as though it is relevant.
Interviewers are people too, you know. When you talk to random people (someone you don't know) about stuff, they are not usually all that interested that they will desire to delve too deeply into what you say - they'd like to just get the gist. This is a psychological observation that I have made which I feel can be made use of.
E.g. during the interview if you are asked about some previous work or school project, you can usually get away with giving a summary and even if at some point you end up waffling they won't notice too much because during your answer, they most likely will start 'zoning out' which gives you some leeway in that even if you end up chatting BS, they won't realise as long as you are doing it in a confident manner and making what you say sound important.
Overall though, believe in yourself. Have strong trust in Allah, too.
If you are confident that you have the skills to do the job and that it is a job that you really really want, then don't be half hearted. Give it your all, your 100% in order to convince the interviewer why you should be given the job. Don't be needy, don't beg for a role (I've seen that happen actually!) but show them why they should hire you.
On a different note, someone told me that the following has always worked for them in terms of job etc.: wake up for tahajjud, pray and make long dua asking Allah for a job and be totally explicit whilst requesting the job, i.e. not just 'Oh Allah, please give me the job that I am having the interview for tomorrow' but 'Oh Allah, please give me the job that I am having the interview for tomorrow,
I really need it because I have a family who needs looking after and there is no other person that can provide for them at present'.
Try to give it a go, please.