jogym you have a bit of a defeatist attitude which is totally understandable. Your cv definitely doesn't need to look 'pathetic' just because you haven't hopped around loads of different jobs. During 20 years you will have worked for different bosses, you will have improved your skills, you will have been involved in different things, you will have taken on different responsibilities and will be able to demonstrate loads of experience in the things they need.
If you have a job description for the job you want to apply for, look at what you would be expected to do, and what skills and experience you are expected to demonstrate. Tailor your cv accordingly. Most people have to be really selective about what they can put for each job because they are trying to squish loads of jobs in. You have the luxury of being able to take as much room as you like to make sure everything relevant in your current job goes on your cv.
Make sure you write in short sentences, use bullet points and good spacing so it's easy to read, don't write in long paragraphs. A 20 year record with one firm is very positive in lots of ways, don't underestimate the skills and experience you have gained over that time, you have an awful lot to offer a new employer.
Just on your second point, I'm not suggesting you change who you are, but it's not that surprising that secretaries who are 'all smiles' and socialise with the relevant people get the good work. It's not right, but it's not surprising - networking and an outgoing, friendly attitude can go a long way sometimes. It also sounds as though you are overlooked a bit simply because it's possible to do so - you are probably seen as someone dependable who 'won't mind' having her appraisal delayed, or not doing the more important work. It's probably easier to avoid upsetting some of the others who might make more of a fuss.
None of this is right, but it is reality. Don't change yourself as such, but being aware of the reason for things always helps you work out the best way to address it. Perhaps in your next job, try and go out occasionally, and have the confidence to realise you don't need to get walked over. Just getting a new job should give you extra confidence hopefully, so should help this.