After playing electric bass since grade school (I'm now 59), I decided 3 years ago to transition to upright. I put off playing arco for a while and then bit the bullet. I was shocked how little playing electric bass helped with learning to play upright. They are two totally different animals. But my upright work has certainly helped my electric bass playing. With a full time job, electric bass gigs, going to my beach place and family I don't practice nearly as much as I should. But I starting taking lessons 3 years ago and progress has been very, incremental with lots of plateaus that last a long while before I see my abilities progress. Learning to play upright is simply hard, and thems the facts. Learning to playing arco was equally difficult. I tried French grip and simply could NOT do it. So for me, I am a dedicated German grip guy. I actually use an Alsatian bow with a frog that looks like a smaller German grip that can also double for French grip, should I ever feel the urge to do it in the future. I had it made by VingoBow on eBay from China (about $350) and I love it, and it is well balanced for either German or French grip. I sold my student French bow with no regrets and bought a second Alsatian.
At first when learning arco my teacher was always asking me if I had enough rosin on, if I torked the hairs too tight, and to RELAX! I still get told to relax and not just for arco technique. Play in front of a mirror as suggested above to make sure you are bowing the right spot for the notes you are playing and for judging if you have good right hand technique. I am told by my teacher and others that adult learning on upright is slow and par for the course, but the more you practice the quicker the progress.
I feel that my arco is finally getting substantially past the screechy/scratchy stages, and that took about 2 years of work at the pace I am going. I still have lots more work on arco to do. Being able to reliably slur as written can be still very challenging for me depending on the piece and/or passage.
Playing pizz is so much more easy for me. I don't have any gigs that require me to play arco. I decided if I am going to play upright that I wanted to learn how to play arco properly so one day I might be able to feel confident enough to play out on arco. Right now I am not confident enough to play arco on gigs, just pizz. I am thankful I have an acoustic duo to play upright in with a guitarist doing acoustic blues in the Piedmont style. We play gigs at breweries and wineries in VA outside DC.
Learning to play upright is hard and takes time...practice, practice, practice...don't give up...and maybe try a German bow...German was significantly easier for me to learn than French. My hand and wrist just could not bend the way it should for French.
This is just my experience as an old man trying to learning new tricks...your experience may be different...I retire at the end of this year and plan to focus much more on my music, including spending more time continuing to learn how to play upright.