How do you stay motivated in a thankless job?

First, you have to like what you do. Which service won't matter. If you need to cross train shop around first. Have your First Sergeant make arrangements to visit some shops you might be interested in. Try one on for a day and pick their brains. Don't jump unless you know what you're in for or what you want.
Good luck with your decision.

I recently cross trained from munitions maintenance to Command post (command and control). I completely understand what it feels like to be unmotivated and in a thankless job or at a least a job that the thanks feels sarcastic, and never seeing the end result of your work. My advice to you would be to first determine if you want to stay in or get out of the Military. Do you not like your job or do you not like the military? If the first, find what you like to do and then try to cross train into it. If the latter, find what you want to do outside of the military and work toward preparing for that if you are not ready already.

Like the individuals wrote below, it really does help when you have a team of people that you really enjoy working with. I know from personal experience how that can really change your outlook on your job and life. Does this mean that I don't notice things in my new job that could be better? Of course not. But they pale in comparison to what I came from.

Don't let anyone tell you what you should do with your future. Take advice from everyone but you do the research and find what you want because at the end of the day when the lights go out, you are the one that has to face it all over again in the morning.

When you look at the list of jobs on MyPers to cross train into find ones from the list that you would be interested in. Only choose those that you know you would like. Interview individuals. Ask them what is the good, the bad, and the ugly about their job. You want to know what they love and what they hate about their daily tasks. Weigh your options. For my job interview most of the complaints were that it was long hours and they were stuck inside with no windows. Compared to what I had at least I wasn't facing twelve hours in the snow holding a shotgun and flak gear.

What do I wish I had done differently? I wish I had been able to talk to the airmen one-on-one about what they didn't like or did like. I didn't realize at the time that they did not want to tell me the full truth in front of their superintendent. I wish that I had been able to shadow someone for a week on the job. So far those are the only two things I can think of that I would have done differently. Do I regret my choice? No, honestly I don't. Even though I have recently been going through a rough spot with training and everything else on my plate, in the end it has really been worth it. My outlook on everything is so much better. I don't come home hating my job. I have a great team that I work with day to day. My supervisor is the best supervisor I have had. The options for this job are endless compared to my last job.

Meet with your career advisor and get to know what options are open for you. Kudos to you for looking ahead for the next two years. Keep your chin up.

There is a reason the "tip of the spear" is pointy. Not everyone can fit on it. The spear is made up of the head (blade) and the shaft. Without the shaft, a spear is just a blade lying around. But it takes both parts of the spear AND someone competent to throw it (or otherwise employ it) for there to be an effect. Not everyone is going to be able to see the end result of their part in the operation.

You have to know that when you do your job to or above standard, it allows others to do their job, and eventually accomplish the mission. Sometimes it's tough thinking that what you're doing at any particular moment has any real effect on The Big Picture. I've been there; covered in oil and grease, reconnecting fluid lines to a T-55-GA-714 engine on a CH-47 Chinook, and wondering why I was doing everything I was. But you have to look at things at your level. My epiphany was when the CH-47D helicopter that I had worked on (taken apart, inspected, put back together) went out for its maintenance test flight (MTF) and it didn't fall out of the sky. You have to savor the accomplishments that you have where you are.

The ordnance techs that maintained, housed, and secured the MOAB that was dropped on ISIS weren't there to see it detonate first hand. Yet, they know that their efforts led to a significant achievement in that part of the mission. You have to have that mindset.

But take the others' advice and if you choose to cross-train, know what you are getting into. Sometimes what you see as a "better" job turns out to be worse than the one that you just left.