Me and my girlfriend broke up abut 6 monthes ago but i still lover her with all my heart?

I wouldn't get her a promise ring without talking to her about it first. What if she read something into it that you didn't mean? If you aren't dating I would hold off on the ring but you could get her something else like a necklace (if you're into jewelry). Just make sure it doesn't come across as a bribe to get back with you. That would be lame.