Remember the old saying, two wrongs don't make a right. Going for the bestfriend will make you just like your husband and even worse if you don it out of spite.
This is not the time to jump into a relationship with the bestfriend. Grass may look greener when comparing it to what you have. Decide if you want your marriage first. If you are done with your marriage then consider someone else. Also, be cautious, the bestfriend may have come back because he knew of your husband's escapades and was anticipating an affair with you. If so, he knew you would be vunerable and open to receive his advances. Just because he acted surprised to hear your feelings for him doesn't mean that he was. The same wisdom that made you suspect your husband of infidelity, use it for this guy also.
You didn't say that you don't care for your husband anymore. Only that you have feelings for this other guy and basically fed up with the cheating and being alone. These are not legitimate reasons to get involved with your husbands bestfriend. They are reasons to know that your marriage is in trouble and problems need to be worked out. If you can't work them out or don't want to fix your marriage, file for divorce, then go get your man.