Feeling Overwhelmed by My Mistakes - As-salamu alaykum

As-salamu alaykum. Lately a heaviness has been sitting on my chest, and it's tied to the mental health struggles I've been going through this past year. Recent events have only made things worse, and I keep telling myself I'm deserving of punishment for what I did. It started with an awful rift I caused with one of my dearest friends - a Break that was entirely my fault. I replay those moments every day and regret them deeply, yet there's nothing I can do now to fix it even though I miss her a lot. Before we fell out, a stray cat used to come to my house daily. I fed it, but it was thin and showed signs it had been hurt before. My heart wanted to take it in, but I couldn't because of the situation I explained to my friend. We agreed she would give the cat a safe, warm home before winter. Even after everything that happened between us, we had promised that if the cat returned, I would tell her right away so she could care for it, no matter how things stood between us. The cat did come back once, and I didn't let her know. Shame froze me, and I was too afraid to face her or speak up. Now, months later, I'm tormented by the thought that my silence might have led to that animal's death. It isn't just hurting my friend that breaks me - it's the real fear that my inaction cost an innocent life. The guilt and shame feel crushing. I barely raise my hands in du'a anymore; I hardly pray at all. I feel so unworthy to ask Allah for forgiveness or to call on His name because of what I did. The weight of it makes me think I don't deserve mercy. I feel lost and alone. I can't sleep, I hardly eat, and I do risky things sometimes as if I'm waiting for an end. Please make du'a for me. I know I must try to seek forgiveness, make tawbah, and do whatever good I can to make amends, but right now I'm trapped in this guilt and don't know where to start.