Sorry to hear that the romance has sort of fizzled for your Sims. If they truly like each other then I don't believe hope is lost for feelings to rekindle. One thing I have learned through playing this game is not everything is dependent upon the programming. Yes, there are events which trigger wants and wishes to occur no matter who the Sim, what their trait, age or skill level; these things were written to fire when a certain criteria is met.
Then there is the other side of the game which is often overlooked and it has to do with Play Style (the random element of the game). How you manage your Sims, what you allow them to experience, what you allow them to learn and what you prevent them from learning or experiencing. Every action you make with them, every bit of guidance you give them, they learn from. They form (if you will) their own ideas about events, Sims and other things.
Sure, with mods you can change the value of many things and open up 'options' for actions but this is a surface fix much like when you enable the 'testingcheat' and slide the relationship bar up to the max. These changes only affect what options for interactions (or restriction from) the Sim can do; it doesn't alter them or nullify what they have learned and the conclusions they form from that data.
You know your Sims and their background. Did you put them together? Or, did you allow them to decide if they wanted to explore each other? The first one fulfills the players wants, the latter expands the Sims learning AI and develops their personality. Sometimes, what the Simmer and Sim wants in terms of partners are perfectly aligned and this is great when it happens. Other times, things can fall apart despite our desire to 'make it work'.
Lastly, developing relationships is difficult in this game. Sims can at times be very fickle and relationships can (at times) be difficult to navigate. For myself, I have found it best to allow them to 'navigate' the waters of their personal relationships (especially romantic ones) themselves. Like us, sometimes feelings change, or someSim comes along who is a better fit and other times feelings simply stall and spending time together or in the presence of each other (even if they don't interact) is enough to jump start it again.
As far as forcing them to do anything, that's very easy. Just cancel out the wishes or actions of the things you don't want them to do and choose only the things you do want them to do (or the Sims you want them to interact with). Reinforce the behavior, they will catch on and build on what they have learned. Hope this helps.