I feel my girlfriend puts me last in everything. How can I talk to her about this?

Be open and say exactly how you feel. Talking about something like that is a big thing and she may not take it well so you need to be prepared for that. However, it is a very important conversation to have so make sure you do have it. Sit her down, and explain and don't let her talk until you have finished, get everything off your chest.

I find the best way to have a genuine conversation with your significant other is to ask them when they are available to sit down and talk. Make an arranged time. A time when you can sit down with no distractions (i.e. - no kids, no phones, no television, no upcoming things to do). Set aside an allotted amount of time and stay on subject. State your concerns with ample amount of examples and then turn things over to them and allow them to ask as many follow up questions as necessary as this will most likely be new to them. Don't be afraid to give time for them to process your grievance, as long as there is an agreed time to revisit the conversation and complete it.