First, you are NEVER TOO OLD to have a crush!
Crushes are a safe emotinal outlet for affection and good feelings, but when they get out of balance, as this one seems to, then it indicates something lacking in your long-term relationship.
It's really easy ti imagine that this man will provide what is subconciously lacking in your partnership, but reality is, you will simply inherit a new set of good feelings, along with an (as yet unseen) set of bad ones, because that is what adult relationshps are about - as previous responses confirm.
You need to see this situati0on for what it is - an idealised imagination which will not be borne out in reality. You must try and focus on your current relationship, and try to find out what is missing. Somethng is, which is why you are attracted to someone else whom you feel will supply it = whether or not he will is not important, it's your need that propels you towards him, so find out what it is, and fix it.
Usual issues are - in no particular order - interest, attention, novelty, newness, ideas or tastes in common, affection, and a (unrealistic) projection of a future with this new person.
Then think - you had all these things with your boyfriend. Some you cannot regain because they naturally decay with time, but some others, the attention and affection, are merely missing, but not lost, so try and find them again.
When you have got over this - and you will - you may look back and be glad you didn't pursue it. Your relationship may break up anyway, but be assured, this crush is a symptom of an issue in your feelings for your partner, it is not a solution to that issue. You need to find that for yourself.
Good luck.