Is it normal to drastically change after a breakup?

Personal change is a thing that's very hard to measure, because it's hard to know whether changes in behaviour are situational (just as a result of not having a person around anymore) or fundamental (a change to your self). I don't know whether that's a distinction which is interesting to you or not, but it's a distinction I would make if I was doing the one doing your thinking.

A lot of people who know me have said I've changed a lot since my break-up, but I don't think I've changed so much as the things I do have changed. It was over a year ago now, but it doesn't feel that long. I do things she wouldn't have let me do, or would have disapproved of, or would have teased me about, and I do them without fear or nervousness. Admittedly your relationship might not be comparable, because my relationship wasn't normal to start with (not because it was special and amazing, but because it was shitty and abusive).

What I'm saying is that I think whether there is much of a change in your life (besides changes in the day-to-day routine you get from losing a close person) depends on the intensity of the relationship, and the nature of the change depends on the quality of the relationship. Every case is different, but I don't think change is abnormal at all. You've lost a person from your life who was very important to you, and I think that means you're very likely to change the things you do.

Yes.

When a girl I really cared for broke up with me years ago I stopped smoking dope as I realized I had been favoring spending time with it rather than her, despite the fact she was a smoker too.

Before this I was smoking several grams a day, dealing, hanging out mainly with other users, dabbling in hard drugs, etc.

I stopped smoking the morning after we broke up & haven't looked back since.
My circle of friends has changed (although now much smaller) & it has basically been one of the biggest changes in my lifestyle I've ever had.

If only it had been a little earlier we might still be together...

Hmm. It depends on the situation, but I don't think it's terribly unusual. I changed a lot after getting out of one relationship because the guy I was with was really controlling and manipulative. I gained a lot of confidence after breaking up with him, and I finally felt like I could be myself again. So it's not just that I changed – it's also that I was suddenly able to express myself and the internal changes I'd gone through over the year that we dated.

I think that can also happen with relationships that aren't controlling and abusive. People often change themselves (in both healthy and unhealthy ways), their behaviour, and what they focus on based on their partner. There are things I enjoy doing with my husband that I might not do anymore if we were to split up, because my enjoyment of them has a lot to do with his company and our shared interests, not my interests alone.

Also, it's normal for people to reinvent themselves sometimes, especially young people, and a breakup can trigger a need for change. A lot of emotional changes and psychological growth (or damage) often follow a breakup, so it makes sense that people are sometimes very different afterwards.