Before you add ANYONE to your life, you and your husband have a lot of work to do on yourselves as individuals and also as a couple to address the problems in your marriage. I suggest therapy or at least counseling; you'll make a lot more progress, faster, if you have a detached professional guiding you through the process.
Once your marriage is stable, sane and happy, you and he can open the discussion on the possibility of adding additional partners or lovers to the mix.
But you MUST address the issues that face you, if you want to find your way past them. Ignoring a problem doesn't solve it. That's probably not what you wanted to hear, but it is the truth.