Is there something I could do considering I'm not really happy with my master's thesis?
Have an honest conversation with your advisor.


  I appreciate the support you have been giving me, but I have to be honest that I'm getting increasingly frustrated with my project.  This isn't the area that I hoped to work on, and I'm struggling to really get into it. 


Note that this is nothing against your advisor.  Be careful not to frame it as a criticism of your advisor; it's about this project's fit to you.  If you put it this way, it shouldn't be offensive.

Also, presumably your interest now is also your interest for your career path in the future?  If so, I would highlight the impact of that:


  In addition, I'm hoping to go on to a [PhD/career] studying [fundamental physics--insert the appropriate term here.  I'm not a subject expert]. I'm worried that I'm not developing the skills and expertise I need to move in that direction.


In my mind, this is the most important point.  If it's just a matter of enjoying your current project, sticking it out for the next year might be the best option.  But if your current project is limiting your future options, it's important to address.

After sharing your concerns, what to do about it?

If at all possible, I would try to explore with your advisor whether your project can be tweaked to better fit your interests.  Could it focus more on theory rather than computing examples?  Could it bring in some other related thing that is more what you want to look at?  Again, I'm not a physicist, so I can't help you on the specifics. The cost of changing projects would be high.  It's preferable to modify the current one if at all possible.  

It sounds like so far your advisor hasn't been very flexible.  However, given that you accepted a project and worked on it for four months "just to be nice", it also seems likely that he doesn't know how you really feel about the work.  Perhaps he will be more flexible once he understands.  

If there isn't flexibility, then you have a tough decision: take the "safe" route of completing your current project, or try to switch projects (and probably risk delaying your degree).  But I would try this as a first step.