Let it all out. This is a time for grieving and it's okay to mourn the loss even if your partner was less than fair to you. Cry it out. Let yourself feel the hurt so you can begin to heal.
You will inevitably fixate on this loss. That's okay. This is part of the process;however, you want to make sure it does not turn into an obsession. Pick a couple of favorite pastimes and do them, even if you don't feel up to it. Hit the gym, join a yoga class, learn to bake, read a classic. Your partner hated when you did (insert activity here)? Well, this is your time to do that activity. Our thoughts can influence our actions but our actions can influence our thoughts and moods. So like Nike says,"Just do it!". You'll find yourself soon thinking about something other than your ex.
Build an emotional support system. Talk to your friends or family. Join a meetup or support group of like-minded people who are going through the same thing. Get yourself a therapist. Does therapy sound expensive? Check out openpath collective (https://openpathcollective.org/), where you can find good help for around $30-$50 per session. Dealing with this alone will only make you feel more isolated, which could lead you to an "Ex relapse". Reach out. It truly takes a village.
Look out for Part 2 soon