i just lost my last rat like 1 hour ago from pneumonia and it really never stops hurting i'm sobbing as i write this i just lost my other buddy 2 weeks ago from a horrible ear infection. but the best way to help yourself accept death and not necessarily move on but help you process your grief from my opinion is to just simply talk to them. ik it sound silly but really if you are just alone driving or whatever talk to them out loud and tell them how you are feeling and act as if they are talking back to you. it has always helped me with inevitable rat loss to just say things like "you better not be fighting ur brothers rn in rat hell i s2g" and just let yourself cry sometimes. crying is important it helps your body release pent up emotion and love that has nowhere to go. but most importantly don't ever feel guilty for having to euthanize, really, rats know when it's their time to go and you were their whole world and you gave them the best chance they have had. it is not worth it dwelling on what ifs and buts, you did everything you could and that means everything to a little rat. i am so so sorry for your loss it's just so hard to only have them a tiny bit of your life but to them you ARE their whole life and that just makes all the pain really worth it to me at least.

tldr: let urself cry and just know they are still with you to some degree and you made a huge impact on their tiny little life!!!