Hello iloveonions, and welcome to the forum

! I`m sorry to hear that there have been difficulties in communicating with your partner, but I`m glad you found us and decided to post. You are amongst people who `get it`. I will get the ball rolling by making a few points myself, and other members will also be able to chime in.
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First, I read in your post that you are feeling worn down, and burned out. That is understandable. Arguing in circles can be so draining. It is commendable that you want to be a reassuring presence for your partner, but that starts with showing up for yourself. How do you take care of you? Do you have any support, hobbies, self care activities that allow you the much needed `recharging` period that you need? You can count this support group as one such outlet of course

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Second, you ask about validation. I myself am learning how to practice validation. Thankfully, there are many opportunities to practice, not only with a partner, but with coworkers, family members, friends...even with yourself! All that to say, it`s a skill like any other, that can be improved. Here is a link to an article on this website about the topic :
https://bpdfamily.com/content/communication-skills-dont-be-invalidating . Have you had a chance to read it? What are your thoughts?
To summarize my points, navigating episodes and arguments with your BPD partner is a two-fold approach : taking care and showing up for yourself, while also utilizing sometimes counterintutive approaches and skills that can be learned and improved upon. Then, when all that is said and done, you can let go, knowing you have tried your best, and that ultimately a dynamic is between two people, and that not all the responsibility falls on your shoulders.
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1. Would I just ignore my feelings if I am upset about him falsely accusing me?
2. I really really would be lost as to what to do after validating his feelings. Let's say he says "yeah that would really suck" then what?
3. I'm afraid he would say I'm being unhelpful by just mirroring what he says, he did that in the past. When he's in these moods, he wants change or an apology from me. Maybe I'm doing validation wrong?
4. I'm afraid that if I validate him, then he'll get angrier and angrier with me. If I tell him something like "yeah it would really suck if your partner would go behind your back to cheat on you", I'm afraid he would take it the wrong way and actually get even more scared or think I'm actually cheating on him.
So, to answer your questions :
1. I think, on the contrary, you would need to be aware of your feelings in order to grasp how they may be affecting you in the moment. Your partner may not be able to imagine how what he says impacts you (it may be beyond his emotional capabilities). That goes back to my first point of having support and ressources in place for when you are upset. Remain true to yourself, always!
2. and 3. I think you`re on the right track with your initial statement of "that would be very upsetting if your partner lied to you by saying he's just not remembering", but perhaps you could be more specific. We want to find a validation target that is closest to the other person`s emotional epicenter. Something along the lines of "I can understand that you`re upset because you believe that I am lying to you about not remembering - it makes you feel like I am purposefully neglectful." People with BPD have higher validation needs in general, and sometimes things that seem `obvious` (everyone forgets things sometimes!) can be read into and spun out of proportion in their minds. Keeping that in mind, after validating your partner`s feelings, you could offer reassurance.
4. After all is said and done, it`s okay to give yourself a time out and come back to the conversation instead of continuously engaging in it if you feel like it is escalating.
What do you think? As examples of arguments come back to you, feel free to share them.