My long-term companion is acting very strange. I think it has to do with paranoia. I'm told that is a very hard problem to resolve. Advice?

Does not sound paranoid but more like a delusional thought that's gotten stuck in your partner's mind. Any other recent behavioral changes? UTI?

You don't say how old your partner is, but in your profile you say you're 77 so assuming he is similar age. You don't say what gender, and this only matters because problems like thyroid and UTIs are more common in elderly women. That being said, you need to help your partner get in to see a doctor to discount other medical issues (like high blood pressure, diabetes, tumor) and this will narrow down if he has the beginnings of dementia.

Your profile also states that your are fighting a nasty cancer in your brain. All the more urgent for you to rally family, friends, neighbors to help you work out a care plan for your partner and you. Caregiving can be very stressful and you need to have as little as possible as you go through your treatments.

It will be very important that you and your partner create the legal protections and assign trusted people to manage your affairs when that time comes. Please consult with an elder law attorney and even a Medicaid Planner for your state. You both need to assign PoAs if you haven't already -- and it can't be each other. Now it needs to be someone outside your relationship. Also very important for you to create an Advance Healthcare Directive (Living Will), Medical Representative (do not assign your partner), and a Last Will (least important for now).

As for your partner's delusion, try not to reason with him, instead divert the conversation or use a distraction. IF he has dementia he is much less able to work within reason and logic. In his mind it was real. Best to do whatever it takes to keep him calm and peaceful. It will be hard at first since we all react to our LOs as if they are their prior selves. Take note if he seems worse in the mid to late afternoons (Sundowning) or wants to keep you in his sight at all times (Shadowing). The priority is to get an accurate diagnosis, at the same time get the legal paperwork in order. Spend the money -- it will be worth it -- or find it DIY online at Legalzoom.com or Rocketlawyer.com (not as "airtight" but definitely better than no legal paperwork and much more affordable. Only do this if there are no siblings or exs or anyone who may contest or question anything).

Keep returning to this forum for guidance and insight as you continue on this journey together. Blessings to you, may you gain better health and peace in your hearts.