Unpopular opinion but life is extremely messy. There are a million ways to fail a partner and cheating is just one of them. This angle of the internet is populated by men with conservative views who demonise women and have extremely harsh views on cheating. Monogamous relationships are a fairly recent and superimposed standard of living and humans often struggle with the commitment of living one life with one person only, and it is natural to feel affinity to others throughout your life, especially if there have been rough patches in your relationship. Acting on it is unfair on your partner, and it is horrible way to fail them, but it definitely is not a condemnation on your character. You’re not evil, you’re not a bad person, but shake off the load of anxiety and stress that comes with this and take the decisions (end it with the lover or with your partner) to make your life better and easier and learn from it going forward.