How can I stop letting that person or the memory of that person dictate the quality of my life?

From my experience, letting go is a hard step. What really helped me was listing ways in which I were better off without them and that they had left my life for a good reason. Every closed door open a big and brighter one. In case of bad experience, think about how they helped you grow or what you've learned from it. Some of my worst experiences I would never take back because it has made me a more open and understanding person.

You can try seeing every thought as a cloud, as cliche as it is, watch it exist, know its there and just let it be. Don't dwell on it and they'll just pass by in time.

It's worth remembering that the people around us usually don't force us to do things; when a thought or memory is dictating the quality of your life, it usually means you are choosing actions that surrender the quality of your life. Taking responsibility can feel hard, but it leads to a lot of freedom. When the thought or memory of that person comes up, you can note or label it "thinking," or "feeling." Then let the thought go and return to the breath. It's a practice that takes repetition to become solidified, but eventually, you realize that they're just thoughts - they can't harm you or make you do anything. You still get to choose based on what your preferences are and what outcomes you'd like; not based on jarring or painful thoughts that can come and go.

Reflecting upon those memories and moments with that person and understanding how he or she changed your life is part of the healing process, and it feels like that person is controlling you, and influencing you to be someone you don't want to be, but really, it's your mind thinking about those times and coping, and sometimes the thoughts that come into your mind are the doubts, the pressures, but they are there to help you find your path again. It takes the dark times to be able to find one of the brightest lights, and eventually, the memory will open a new door in your life waiting to be filled with happy memories, and that person will no longer affect you anymore. It's the process of being broken, healing, and starting fresh.

I suggest you embrace this lesson instead of fighting it. Embrace what you've learned, find the pattern: have you discovered how do you want to be loved? Have you learned the right way to feel nurtured? Have you given more than expected? Then you've got a treasure chest of emotions and assets that can be replicated. This doesn't mean you should erase this person or those memories, but you can try to turn them into your personal code. Take care of yourself and keep your eyes open to find someone at the right moment: you will know it when it comes along.