How can I prevent depressive reactions?

In another question, I wrote that despite my resolutions, I had sex without a condom again, and now I'm worrying that I might get HIV as a result. A lovely user chatted with me privately afterward and told me to forgive myself.

Unfortunately, I somehow can't forgive myself, even though I should. There's nothing I can do now; it is what it is! Unfortunately, it's really getting me down because I haven't stayed true to myself, and instead of making the best of it like he is and focusing on the rest of my life, I'm spiraling deeper into depression. How can I counteract this? I should use my energy, move forward, and not mope around. Other people would be happy after sex instead of just hanging around feeling sorry for themselves.

How do I just get this topic out of my head? I feel alone, confused, guilty, and bad now; it's weighing on me… I've even talked to real friends about it, and they just told me I should just do things differently in the future, but that doesn't resolve my feelings of guilt and anxiety…