What are the chances of a girl getting pregnant if I did not "finish"?

Some exist. You have expressed some ejaculate (come) even if you did not "finish". The risk is there.We call people who use withdrawal "parents" way too often.Use a better method of birth control.
Pretty good. The excited penis secretes semen that contains sperm constantly, not just "when you finish". Since it only takes one, over time, with this pattern of sexual activity, pregnancy is a likely outcome.