My girlfriend doesn't pay me much attention anymore but we're still very much in love. I've always been interested in being polyam but she seems less enthusiastic. I feel like this could be the way to save our relationship but am I just stringing out a painful end?
Opening a relationship does not save it. If you're already having problems, those will most likely be magnified and exacerbated by the opening of the relationship.
The solution isn't to find someone who can pay you attention and fill in the gaps of your unmet needs - but to talk to your girlfriend about the issues you're having. Be as specific and actionable as possible.
For example, "Please pay more attention to me" is a lot less helpful than something like "Last night, when we hung out, you were on your phone the whole time. I feel like we aren't spending as much quality time together as I'd like lately. Can we plan an evening soon where we give each other our undivided attention?" or "For a while now, it's always me who texts you to make plans and takes all the initiative - can we talk about what's going on? It's starting to bother me and I'd like to know whether you can commit to doing more to show me you care."
Think also about whether something has changed in her life that might be taking her attention away. If she moved farther away, got a new job, has been sick or especially stressed, etc. then you might have to approach things a bit differently. Finding out how to get your needs met without asking her for what she can't give right now may take some creativity and compromise as well as open communication.
Work on the "attention" problem first. Either figure out how to re-right this listing relationship, or identify what your dealbreakers are and commit to ending the relationship if you're not getting what you need. Once this has been addressed (one way or another), then it'll be a much healthier time to start thinking and talking about your own interest in polyamory.