Well, don't fool yourself: violin is difficult. But...
Personal experience: I was (at the age of 15) in a similar situation as you are now. Knew guitar, wanted to learn something more classical. I tried violin. And tried. And failed.
Actually I didn't try that long, nor did I take professional lessons. But I really couldn't see it going anywhere. The thing with violin is, not only is is hard to play good, it's in fact hard to produce any sound that's not plain painful. And even worse, after an hour of practising, that horrible scratchy mess kept following me around in my head all day long...
I sometimes regret not having had the willpower to learn violin. But well... I ended up learning cello instead, which turned out to be rather closer to guitar in many a sense, yet still can do many of the things that are nice about violin. Something you should consider, too! I'm very happy being a string player:
- Violin/cello are orchestral instruments. Don't know if you had that in mind, but playing in an orchestra can be an amazing experience. It's a way to be part of a truely, well – great sound, without necessarily having to be extremely skilled. (You may get a similar experience with electric guitar in a rock band, but that never has the kind of “part of something huge” feeling.)
- Since an orchestra needs quite a large number of string instruments (unlike winds), it's relatively easy to get into one – provided of course that you can play decently, which as I said sure isn't easy. In that regard, the best choice would however be viola or double bass, which tend to be most sought for (because so rare).
- Keyboards are of course also used in orchestral settings, but unless you're really good and can play piano concerts, you'll end up mostly with somewhat silly (but often crazy convoluted) celesta rings, or kitschy fairytale clichees.
- String instruments can be used in a remarkably wide range of different ways. The typical silky orchestral violin section is only the tip of the iceberg; you can also do fiery dramatic soloes or rumbling rythms or spooky effects or breathtaking innocent folky tunes. Of course, piano is also versatile (at least keyboards in general), and guitar can achieve a lot through FX processing. But IMO there's no class of instrument which offers such a wide, immediate and emotional response as strings do.
- And they manage to fit in pretty much any musical context, if played accordingly; you're not at all bound to orchestra playing.
- Especially violin is nicely compact. You can take it everywhere, practise anywhere, and join any session. OTOH, piano... let's not start.
- The bowed string instruments, together with guitar skills, readily open up many other instruments. Mandolin is really pretty much just strummed violin with frets, electric bass is exactly the same as cello, and the intonation wisdom can be translated to all kinds of instruments, as well as singing. With piano? Well, you get the other keyboards almost (but not quite) for free, but keyboards are utterly useless for many dimensions of musical expression.
So: definitely try a string instrument, if you're willing to go through some efford!
To disclaim: I'm frankly a bit of a piano hater. Of course piano has undeniable qualities, and sometimes nothing is as fitting as a piano accompaniment. But I find piano is way overused, in part surely because it's so much easier to become decent (which doesn't mean it's easy to become good!), and IMO it also doesn't have any benefits over guitar as a harmony / theory device, in fact I consider the 12-toneism that piano induces rather harmful to composition.
You can (and should) always also learn a bit of keyboard in addition to other instruments. But if you want to become a classical pianist, it requires no less efford as for violin, and honestly I'd consider this is wasted lifetime in your position.