Hey! I'll be 21 when I start at Bristol in 2018
Where are you thinking of applying?
PS wise, I found it helpful to write down positive traits I have, examples from work experience (veterinary or not!) where I have demonstrated these traits, then link them back to/pick the ones that fit veterinary as a course and career best.
Also - mention extracurricular stuff. Let them know you're capable of balancing work and fun. e.g. I wrote about my running to blow off steam, and how I ran the Land's End to John O' Groats distance this year, which was a personal goal of mine. Knows how to relax - check. Is active - check. Can make and achieve goals - check. Whatever you do, link it back into to how it makes you someone who
personally suits this choice.
If you're really stuck, try not to write beginning - middle - end; it can be really daunting, and can make you lose track of what you may want to say. Initially just jot down ideas as they come, and gradually form them into coherent paragraphs.
I would say don't stress how much you love animals/science etc as that's something that's more or less a requirement for the course. Instead, just demonstrate it by mentioning your motivation to take science A Levels as a mature (19+) applicant, and the hours of work experience you've put in. In other words, show, don't tell.
Other than that, I can't really think of much else! Be truthful, but don't be afraid to big yourself up.
Good luck!