When I forgive someone, should I strive to "forgive and forget' and not hold on to pass mistakes or should I "forgive but never forget" and keep my guard up?

I think this is dependent on the context and your personal feelings. In my opinion, you can’t force yourself to forget something, and sometimes having your guard up is self-preservation. But I do know that some find it helpful to forget and that it can provide a sense of peace. Personally, I tend to be guarded and assume the worst, which admittedly has had it downsides, such as struggling to build friendships and the mental toll of worrying about people’s true motives. Overall, I think it makes sense to take things at your own pace, and if you reach a place where you feel letting go of past mistakes would provide you with a sense of closure, for example, then go for it. However, I don’t think you should feel bad if you can’t do that right now.