How Can I Reconcile Romans 6:12–14 with Romans 7:21–25?

Questions about how to reconcile Romans 6:12–14 (where Paul talks about victory over sins) with Romans 7:21–25 (where Paul bemoans sins he still commits) and why Paul uses the Mosaic Law as a starting point for moral instruction if believers aren’t under any part of the Law.

• Please help me reconcile Romans 6:12–14 (where Paul seems to be saying we can have victory over our sins) with Romans 7:21–25 (which sounds like Paul is bemoaning sins he still commits).
• How do you reconcile Paul’s use of the Mosaic Law as a starting point for moral instruction with the idea that believers aren’t under any part of the Mosaic Law?