Hi , welcome to the community and thank you for reaching out about this. Sorry to hear that you're struggling with these side effects. It's rotten that we have to deal with these on top of the PsA symptoms! The folic acid is really important, so I'm glad to read that you're taking that alongside the methotrexate.
I've been taking methotrexate for almost six years now and I did struggle with some nausea at the start, and I still regularly feel quite groggy for around 36 hours after my dose. I'm glad that upping your water intake and splitting your dose has helped a bit. For the nausea, what helped me personally was firstly taking it with my evening meal, so that I was asleep for the first stretch of nausea. I'm not sure whether that would make a difference for you if you're suffering for multiple days after your dose. The other things that helped were ginger products (ginger biscuits, non-alcoholic ginger beer, etc) and ice lollies - I'm in the UK and not sure what they're called in the rest of the world, but I'm talking about flavoured ice on a stick.
Have you had a chance to discuss this with your doctor yet? I ask because, based on what other community members have shared, there are a few things they may be able to do to help. Some people find the side effects are more manageable with the methotrexate injections - if you're taking the tablets that might be something to consider. Your doctor may also be able to prescribe anti-sickness medication, or they may decide that a different treatment is more suitable for you.
I hope you'll get some more input from others here, and that there's a way for you to get the relief you deserve with as few side effects as possible! Please keep us updated if you can, and feel free to reach out anytime you need support or have questions. Warmest wishes, -Catherine, Community Moderator