There are things I could do. I've been looking after myself really well lately, I'd like to exercise more which I could do
This is good. If you identify something you can do (this is what I did, anyway) just do it a little tiny bit. So, if you feel you need to exercise more, don't overface yourself with researching joining a gym and getting a personal trainer and how will I afford it and how will I sort out the time for myself and do I work out better in the morning or evening and what if I get an injury and... etc etc. Do 5 sit ups, now. Right now. Turn yourself into someone who has already started on your self care mission.
Do this with everything. If you think you need to eat better, give one of the chocolate biscuits in your fridge to nextdoor's little 'un. If you want to drink less, pour the last of that bottle of wine in the sink. If you want more interests, join a Meetup group. Do tiny things, but do them now, do one a day. You'll feel the impact very quickly of being in a relationship with someone who's good for you: you. And then other relationships will have to work hard to compare.
My view of relationships that don't make me happy now is 'I can't be bothered with this'. I used to try so hard, and need so much from others. Now I don't really need anything from anybody, so all my relationships (friends/family/partner) are ones that put a smile on my face, because I simply don't bother with the others, and you won't with your ex, either.
Honestly, start now. Do some little tiny thing, just for you, that'll make you feel you've done something good for yourself.