In relationships, it’s healthy to have outside sources of support, so when you say your partner sought outside counsel for a problem he was having, I’m not sure why you’re jealous. It’s unrealistic to think you or your partner can be everything to each other in a long term relationship. So you might do well to relax your ideals of what you mean to each other. You can still have a healthy commitment and have friends you go to for advice or input on issues.
As for your partner’s saved letters from old boyfriends, he’s keeping them because they have sentimental value. That doesn’t mean he doesn’t love you and feel committed to you. Unless he’s reading them aloud every night, I wouldn’t worry about it. You chose a man who has a past — and frankly, most adults do! — so it’s pretty normal for him to want to save some mementos from old relationships, as long as they’re in a box in the closet or the attic or the garage. I’d worry if he was framing old boyfriend letters and putting them on the nightstand next to the bed! 🙄 But since it doesn’t sound like that’s the case, there must be something else bothering you that you haven’t mentioned to me here.
I hope this helps. Let me know how things go. 🙂