I think the main things to consider are:
1) Don't return to the usual "Dance Steps"--in other words, the way it went in the past: you'd discover something, she'd be defensive, you'd be angry, she'd cry, you'd say "him or me", she'd make promises, you'd wait and see, she'd make up to you without actually following through, after a while you'd go back to the same old same old with nothing truly changed. If you do those same steps, then you'll get the same result. This time we aspire to a different, (hopefully) better result! So looking at the steps you usually do, where are you in the "dance" and what could you differently instead of your usual step?
2) Don't go inactive and let this be swept under the rug--so far both of you have essentially avoided resolving this issue for roughly 20 years! And part of the way you two avoid it is by "going inactive"--so this time continue to take one different step...then another different step. Keep moving forward. You can not make her give up a lifestyle of being inappropriately flirty and partying with other men--SHE will have to want to do that of her own accord. But it is absolutely reasonable for you to keep yourself moving in a new direction (doing things a new way) and keep letting her know where YOU are at, and what YOU think and feel. Okay yep keep in mind whether or not you're giving away strategy, but you can share with her what you choose to share with her. Keep moving. Keep growing. Keep doing something different and then if it did or didn't work, adjust and try again. Don't let it slip back into "swept under the rug.
3) Don't try to control her; instead control YOU--part of seeing your spouse as an adult is giving them the chance to be an idiot. LOL What I mean is that not every choice someone makes is going to be wise, but an adult experiences the benefits and costs of the choice they made, an adult accepts personal responsibility, and an adult learns from it and grows and does better. YOU can not make her be a faithful, honest woman. But you can control YOU. You can make choices based on all the facts we don't really have here on TAM. You can make decision and accept both the costs and the benefits of what you purposefully decided. You can be responsible for YOUR LIFE. And you can look back and learn from what you've done before and what you're doing now and say "this worked...this didn't..." and grow. YOU can be a faithful man if that's who you choose to be . YOU can be an honest man too. And YOU can choose to ONLY have a partner in your life who shares those traits. Those are all legitimate choices. So instead of focusing on "making her be honest" ... focus on what you need to change and what kind of man you choose to be, and be that guy. Be... it's a verb, and verbs involve action.