First @Miki, you are a treasure.
Second, yes, you really shouldn't try to fist yourself with your actual fist. In my travels I've run into so many people who think they can jump STRAIGHT into fisting or the heroic-sized dildos and it's just not going to happen. There are many kinds of fisting toys you can try, and I ALWAYS ALWAYS ALWAYS recommend something (I hope links are okay here) like The Stuffer Fisting Hand Dildo.
https://www.google.com/search?q=The+Stuffer+Fisting+Hand+Dildo&tbm=isch&sa=X&ved=2ahUKEwi1xdOkuP3_AhU8FzQIHegjAcoQ0pQJegQICxAB&biw=1426&bih=655&dpr=1.5
The shape here is more of an apex, and it comes in a hand and forearm option for deeper penetration if you're looking for that. Both come with a suction cup so they're ideal for self-play and can be used both anally and vaginally with no problems, and are silicone/water-based lube friendly.
Once you get used to this kind of toy you can move on to a classical fist (the company that makes The Stuffer makes a regular Fist and Knuckles version that you can upgrade to with time, both are pretty decent quality imho!) but I highly recommend mastering a "finger duck" type fist before moving up.
Like Miki said, don't make anal fisting a regular thing because it will fuck up your asshole, that's just a fact of life. For anal fisting I suggest looking at some kind of deep fisting cream/desensitization cream that will help lubricate and numb the area out so you can enjoy the experience and not fuck yourself up. AGAIN, limit it to every once in a while, work your way up to that kind of play. Start with small dildos, think of it like gauging your ears and work your way up! Deep fisting cream isn't required for pussy play in my experience but YMMV.