Your letter seems peppered with so many worries and all I can ask you to do is simply stop doing that. You seem to place a lot of responsibility on what YOU should do in this position when invariably a relationship has a life of its own and it in itself dictate your options in the future.
It appears that you've only recently left Marcus and found Chuck, and quite naturally all the unpleasantness of the past is very much fresh in your mind. You definitely regret the past and want to avoid a repetition at all costs.
Chuck Is NOT Your Ex
To me, it seems inappropriate to contemplate whether you're in the right relationship at this moment. Chuck is NOT Marcus. Chuck has his faults but they don't appear to be upsetting ones AT THIS STAGE.
At this stage? Listen, he's 23, in school and naturally would have a different perspective on life. You're 31, out of an eight-year relationship that has been traumatic and obviously would look at things differently. Why don't you concentrate on what the TWO of you want AT THIS STAGE and enjoy life?
Give Yourself Time To Heal
Give yourself some time to heal and surmount this event in your life and then contemplate whether this is the best relationship. A relationship has a pace of its own; contemplate the "happily ever after" conundrum at the appropriate moment.
It seems you stayed with Marcus because of the security of it all.
Having given some time to get over all this, when it comes to that moment when you must think of the future; if for once you think you're in it predominantly for the sake of security then you might want to ditch this and have the courage to seek out your soul mate.
He's Young and He Will Change
But then here we are postulating over the future when all this can change in a flutter. Remember Chuck is 23, and a student. He's young and life changes when he graduates, so...
You Looked for Someone Similar
On a final note, it's quite natural Chuck resembles Marcus because you've shared so much time together, you would look for someone similar (Marcus is a sort of "mental imprint of a "suitable" partner). I always rely on my instincts about whether this is the "one" and perhaps your instincts will tell you.
But having said all this, avoid "being there" for Chuck at all times, expect your quid pro quo so that you could avoid a repetition of the past.