Sticky situation please help!

My ex wasn’t stepping up to the plate so we broke up. 3 weeks went by and I figured well he hasn’t shown me he’s changing or making an effort. So I met someone and we made out that’s it nothing bad just kissing. Well now my ex comes back and has been doing all these things to better himself and he wants to wait to officially get back together until we fix our problems but he doing everything he can and needs me back he doesn’t want anyone he hasn’t seen anyone else. I really believe him and now I feel like shit for kissing someone else. The guy I kissed also just got out of a relationship and I’m feeling like a rebound after some things he said to me. So now I’m super confused and I’m afraid that I’m going to loose my ex if I tell him but I also don’t know how I can keep it a secret. I’m nervous if it’s right to get back with him. It he seems really serious about fixing and changing everything. And idk how to end things with the other guy. I’m so confused and stressed out and feeling horrible about myself someone please help and give me advice