I believe any marriage is salvageable, even marriages with infidelity and God will use all things to his glory. I know this because I have been in your situation, take this time to focus on God and yourself, pray for your marriage, your husband and as much as it hurts pray for his mistress.
Many people will tell you to let him go and to move on only you and God know how much you can take and only God knows what he has planned for you. Read Jerimiah 29:11,

Don’t be so hard on her, negativity begats negativity.