It sounds like the same issues most of us have had while learning to play, you are part of a great club!  

I don't have the answers to anything, and take anything I say with a grain of salt, but I will share what I have found works for me, largely coming from my own experiences as a guitarist, and I have applied those experiences to other instruments.

1). The instrument itself.  I recently saw a statistic that was something along the lines of:  90% of people who spend $1,000 or more on their first violin keep playing, while 30% of people who spend $200 on their first violin keep playing.  But, that's just a statistic, and it does not measure a whole lot of other factors.   On the surface, it could be read as "if you spend more on a violin, you will be more likely to keep playing"

What's more likely going on though, is those who are more committed AND HAVE THE RESOURCES spend more on their first violins.  A very big difference there.  Since you are posting here, and listening and responding, I think you have that level of commitment to yourself in there.  You may stop for awhile, but you will go back to it.  I have done that from time to time over the years on various instruments.  I always go back, because I can't NOT play.  What I have learned over the years from that is, I don't beat myself up by thinking "If I had just kept playing X hours every day over X years think of how I could play now!"

I used to do that a lot.  Now I just try and get to the point of enjoying where I am at, and progressing from there.

Back to the instrument itself, I do have some personal criteria there.  It needs to have a look that inspires me to want to pick it up and play, and it needs to have a sound that makes me want to pick it up and play.  For me, electric guitars are easy that way. Give me $200 and I can find a used electric guitar that has the look and with another $200 worth of parts and work will play as well as any top of the line Gibson Les Paul at $3500.  Which is still a lot cheaper than in the violin world.

There are probably some gems in the rough out there, and you might be able to find something that has the look, and capable of putting out the sound you want, with some changes/upgrades.  I dunno.... violin is still a bit weird to me though, because bow has so much to do with it.  And I have probably rambled on enough on this part.

2) On mindlessly playing tunes instead of practicing technique AND tension.  I *never*, absolutely *NEVER* play mindlessly anymore.  That is a recipe for disaster for me.  It built a lot of tension, poor posture, poor and limiting technique, nothing good ever came of it for me.   Now, anytime I pick up or sit down at an instrument, it is mindful.  Whether it is sitting down to learn a technique or mechanic, or 'just' playing a song, I try to make it mindful.  What is the difference?  Technique for me tends to be slow and focused on the physical mechanics.   When I play a song, I dive in to the song, and feel it.

The mindfulness on technique/learning is a lot easier for me to describe.  I posted a thread here "Sasha's Secrets #1: Perfect Practice".  That is my approach to that area, and something that I found worked well for to overcome tension that crippled my guitar playing and had to overcome.

The diving into feeling and playing, that is a lot harder to describe.  I think I might have to take a look at that, it might be worth it just to share and understand more for myself how I got to that point.  That sounds like a good topic for "Sasha's Secrets #2" 

No, not that I know everything, but for me, writing and sharing solidifies a lot of things for myself, so I try to keep doing that.

3) Time to practice.  Finally, I never try to carve out an hour a day anymore.  For me, it's too overwhelming.  These days, I bite off a realistic chunk.  5 minutes a day.  And I have found, that is all it takes for me.  I never sit down to practice for an hour.  I only sit down to practice for 5 minutes.

The other part of it is, I have a specific goal for those five minutes.  It might be a technique, a passage in a song, or whatever.  But, it's just 5 minutes, no matter how busy my day is, I can squeeze in 5 minutes on one instrument or another, and work on something.

In reality, I end up spending more than 5 minutes and go longer and accomplish a lot more.  There is the occasional day, very infrequently, where I truly do just only spend 5 minutes, but the vast majority of the time, I end up getting in 20 minutes, 30 minutes, even more in practicing.   I admit, this is a mind game.  But for me, it works well.   So many days, I think "I am too tired for an hour.  I have too much other stuff to do.  etc..."

Giving myself 5 minutes, gives me just enough space and time to get out of all that crud going on in my head to let myself just enjoy and go with it.   And 99.9% of the time, that is all the space I need.

Anyway, take everything I said with a grain of salt, this is just some of the things that I have found work for *me* over the years.  If anything helps you or anyone else, totally awesome to the max!