You're a professional photographer. You're talking to someone who's not a photographer about your job. You've told them how difficult being a photographer is because of competition from the Internet and other photographers. But you are successful anyway. You say this.
But I've managed to do OK.
To "manage to" do something is to successfully do something, even though it was difficult. A few examples:
Aaron managed to get us front-row tickets to see The Rolling Stones. Can you believe it!?
To "do well" means to get a good result. It's usually used to talk about how much money someone has made:
Their software design firm has done extremely well over the last 5 years.
When something has only gotten slightly good results, or you want to downplay results that were actually very good, you say it "did OK":
We did OK in the first quarter, but I think that our second quarter growth will be really great.
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