How do you learn to trust a partner after being in a long term abusive relationship?

Start small. Take things slowly and trust your partner with the minor things in your life before you can trust them with the more personal things. It takes time, what matters is that you keep trying and don't completely give up on trusting people. Maybe have a discussion with your partner first and give him/her a little background as to why you want to take things a little slowly. You don't have to go into a lot of detail, you can just tell them the stuff that you're comfortable sharing. That can also be your first small step towards trusting your partner. You're generally headed in the right direction by not shutting yourself off completely after an abusive relationship. That takes a lot of strength. I hope you keep going and I wish you the best of luck.