I lost my first Betta and I’m so upset

This is a sad story, and I’m sorry I’m advance. Please don’t read or skip to the end for a tldr, as this was a very upsetting death.

I bought Fu, a red male twintail, from my local Petsmart about 5 months ago. I was an idiot and had no idea what to do properly and actually joined this subreddit in hopes of giving him a better life. He was very happy in his tank with his mates but I realized he needed something bigger to thrive. So, for the past week, I have been cycling a brand-new 20 gallon tank in hopes of moving him in when the parameters were proper.

I have been reading so many stories about people losing their pets to different things since I joined this subreddit and I always thought: wow, I’m so lucky, that despite being stupid when I started his tank, that he had thankfully lived and thrived while he was in there. He was less than a month away from a new tank.

Here is where it gets sad, last warning to back out.

Today, when I came home I went to feed my tank and noticed my crab had “moulted.” I thought I saw his shell next to him as my shrimp had just done the same the other month. I grabbed my long tweezers to grab the shell - but it was Fu..

Fu had been living with my red claw crab for about a month and a half- with no issue. I think my error was during the last cleaning of the tank I left Fu’s resting log too close to the planting I have for my crab to climb up for air. I think Fu was murdered, as he showed no heath issues and my boyfriend was in my room when I came home and said he saw Fu swimming around earlier in the day.

I’m so fucking upset and I (not really) want to throw this gd crab out of the window. Fu was happy and healthy and I had a brand new tank waiting for him. I’m so unsure of how to move on after this - I loved him more than I realized.

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TLDR: my healthy Betta male died an untimely and upsetting death when I was in the middle of smartening up on how to take care of him. I don’t know how to move on.