How do you deal with the negative aspects of the job?
In particular the poverty, abuse and mental illness that accompanies your clients
People deal differently. I don't want to claim I've got the answer, because I really don't, but I have a really strong "not my problem" field. Always have. I love helping people with their problems. I love the sense of importance (in both positive and probably negative ways) that comes along with doing something very meaningful for my clients. I take great pride in doing my job well and it would bother the shit out of me if I failed at that, which absolutely does mean going that extra mile at times for clients who are in messed up situations. But at the end of all that, after I've done the best job I can for my fucked-up, drug addicted client ... you just release them into the wild again and the odds are very strong they'll get in trouble again.
I know some people find it very difficult to draw a line. You want to really help. You may be tempted to give them money, emotional support, try to solve their underlying problems ... you just can't. You start doing that and it's an endless pit of need. I do keep a few numbers handy for referrals. That is, I try to make sure that my clients have somewhere to go where there are professionals to help them properly. If they go, great. If not, I can't be responsible.
I'll add two points, which give me some comfort. First, even people whose lives seem to be terminally fucked up may one day turn a corner. It's hard to see it in the moment, but then sometimes you see people in their 50's and 60's who had that life and who straightened out eventually. You can't force it, but you can hope for it, and try to help your clients minimize the damage they do to themselves in the meanwhile. Second, you can at least treat your clients respectfully and take them seriously rather than processing them like cattle. These are people who have a lot of contact with agencies and professionals of various sorts. Giving them good and respectful service goes a long way. It's not just the right thing to do, but it's healthy for them.
Further to the above. Clients with persistent problems (addiction, mental health, poverty, etc.) who are caught up in the criminal justice system tend to string along on all kinds of repeating, minor charges. I hate to say it, but they are the best clients to build a practice on in the early going. You'll never get a big murder trial there, but they keep coming back and coming back and coming back and they never go away for long because their charges are minor. Treating them well isn't only the right thing to do, but a great business practice. They'll come back to you simply for that alone, even if all you can do is plead them guilty again because they were caught red-handed (again).
How does the scope of the work differ from how it really is to what you expected before you got into it?
This almost sounds smarmy, but it really is almost exactly what I expected. I suppose I could say there's more pleading than I expected and fewer trials. I suppose I'm surprised at home much time I spend as an administrator rather than a lawyer. But in real terms, it's what I expected. Because for me, the work is mainly about who I serve and how I interact with them. The law sometimes surprises me, but the law is just a tool. The work is with the people. And the people are who I expected, because I knew those kinds of people already. I think I went in pretty well with my eyes open.