It is unfortunate that your trust in your girlfriend is dwindling. Have you crosschecked the information given to you about her cheating on you and it is valid? I believe she still values the support you gave her.
However, sleeping around with other men and worse still those you call your “best friends” as you claim is fishy. If you love her, and you are not about to send her away because of her conduct, there is need for you to sit down and do an objective self-assessment. If there are some issues which could have been carried forward from your previous relationship, unknowingly or knowingly and could perhaps contribute to such behaviour, then critically analyse them and see how best you can deal with them.
Consult a counsellor to help you carry out an objective personal Strength, Weaknesses, Opportunities and Threats (SWOT) analysis if it is. Remember, it takes two to tango and do your personal analysis and find out where your actions could have possibly created a loophole for your woman to cheat on you. Plan a meeting to discuss at length.
Stay calm. Both of you could do an objective SWOT analysis of each other before the meeting, so that in your meeting you are able to discuss the weaknesses which could have contributed to the present situation.
The best way to do it; would be getting someone else who is neutral to facilitate your meeting or a mature counsellor. Be prepared to listen to each other without counter attacks.
Accept your weaknesses and work on them as you build your strengths.