From my own experience, it’s better to close one chapter before opening another. Ask yourself why you are having feelings for the guy of 10 years. No one but you can really say for sure. If it’s because he is the one you compare everyone else to, it means he’s the ideal man in your eyes and you deserve the man of your dreams. If it’s because he’s the guy you’ve come to know the best so it’s easy to measure guys to him, you need to distance yourself from him until you’ve fully explored the relationship you’re in.
I say, start by putting distance between you and the guy from your past so that you can focus on the guy you’re with. If after a month or two of no contact with the guy from your past you can’t keep him off your mind, your current relationship isn’t strong enough to go to the next level and you need to take a break from it until you know which way to go with the guy from your past.