end of a friendship?

Back in September I passionately kissed my best friends ex boyfriend .. it was something I instantly regretted but happend in the moment. Afterwards we didn't speak for three months and I missed her so much. We made up and are back friends again but I have an insane want to go further with her ex. I am so attracted to him and it's mutual. I have held off since September and not talked to him at all but I cant help thinking about him. Should I be ashamed of myself and try get over it has anyone been in the same situation ?? What should I do .. Also they still talk to eachother on a daily basis but I don't think they will get back she told me she didn't care about us kissing but she literally waited for me to apologise and that's why it took so long for us to talk again