Sometime the game you are playing is simply not the game you believed you signed up for. Happened to me few times, see first paragraph here for an example.
You talked to DM and to the fellow player and it changed nothing. I believe they are trying, at least from your description we can assume good faith on the DM side. What you can do now is to talk to your DM and fellow player and tell them, without accusations, what you told us. You can dress it in words like this:
Guys, when I joined the game I thought we'd all be pirates and I could get away with more chaotic choices. I had the most fun in our one time in combat when we were fighting....pirates. The guys I should probably be allies with. Combat has been the only time I've really felt like I could make some creative choices. In role-play situations, I feel like what my character wants doesn't have any weight.
This is not the game I believed I sign up for. And this is not the game I created my character for. There must have been huge misunderstanding, and now it prevents me from having fun. I don't feel my character has any reason to continue doing what she is doing, and I won't have fun playing her if this will not change, so we need to have this fixed. I guess we can either change the story, or get me a new character, this time one that fits.
Note that I mostly used your own words - you are really good describing things without getting aggressive and accusatory, good for you!
What can happen is:
- Campaign will change. You will work something out to make your character relevant and give him reasons and goals that actually make sense, possibly with minor retcons. NPCs and encounters will give you opportunity to shine and affect the story. You are good.
- DM will help you create character that's relevant and fun for you to play. You'll roleplay transition, or just do it as downtime description of event. Your new character will have his goals and needs that fits the campaign you're playing. Great.
- DM will tell you 1. or 2. is happening, but nothing will really change.
Three is a sad part, but you probably should be prepared that it may happen. In my experience, this is the time your character just sails away* and you stop appearing to the sessions, at least for this campaign. If talking about the issue two or three times changes nothing, it is hard to believe fourth time will help.
I have told my players in Bastion RPG Club things like "Sorry, but I can't make this campaign fun for you while keeping it fun for other players, I tried but I failed, can't help it. Please accept my apology.". I had my character removed from adventures on my own initiative if needed be. Sometimes different campaign in the same group, or different gaming group, may simply be much better fit for you. If that's the case, you need to honestly answer very important questions:
Is it a good use of my time, playing a game that's no fun to me? Am I willing to do it? Why am I doing it?
If you decide it is not a good use of your time, you are not willing and do not have good reason, play your last session, let your character die or ally with pirates, thank other participants for playing. Be polite, but be firm - you have the right to have fun just like everyone else, and you have the right to quit if you're not getting it.
* Actually one of my characters simply retired, and I let two of them die, once even refusing DM "deus ex machina" to keep him alive.