Is it ever truly wise to draw firm or fixed conclusions about your mystical or religious experiences?
There are certain conclusions you can make, such as it really happened, and that is was beyond any normal-level waking experience. You can conclude you experienced something truly beyond anything else you've experienced, and you can conclude it forever changed your life.
Beyond this however, thoughts about the experience, belief structures we use to talk about it to ourselves and others should always remain open. To affix an idea or belief to it and never allow that belief to be modified or even replaced throttles the life out of it. When a metaphor becomes a descriptor, it becomes a dead metaphor, no longer pointed to something beyond itself, and instead takes the place of the mystical experience. Another word for that, is idolatry.
Should you try to arrive any firm beliefs about the reason or reasons you had them, their causes, or any other explanation for them?
It's good to have some framework that is functional and supportive of experience, but "firm"? No. That then becomes a distraction from truly letting the experience be itself and inform you. I do think that when people do that, they are avoiding God. The "true believer" for instance, has no faith which allows for them to not know something. It is a symptom of fear.
Should you try to forget you even had them?
Good God no!
I talked with someone very shortly after my first mystical experience where I was basically on my knees sobbing from the degree of release of my soul that I was experiencing as a result of the experience. She said to me, "Never forget this. Never let it fade." It never has, and I never did. It has been the cornerstone of my entire life's history from the time forward through the decades.
What's the wisest way to handle having had mystical experiences?
Talk with those who understand them, preferably those who have had them themselves - not religious people who have nothing but ideas and beliefs. Mystical experience tends to be outside their wheelhouse of theological beliefs. Experience is everything when it comes to this. No one can tell you the answers, but they can point you to find them for yourself through this. There are ways to return Home, and it will be your path.