I get the problem. It's all too common, but it's really complicated once you're there. So the way to solve it is always to ask yourself:
A. Can I live with the regret if I did not try?
B. Is the feeling going to change? If so, can I live with the consequences?
C. Can I accept and support them with other people? Through children and marriage, without any conflicting emotions?
I've had situations like this and always settled them. Never dated my best of friends and so I never lost any of them. No regrets!
Their relationship was so painful. But it's also painful if TJ were to lose Ian forever. I'm so torn. Could they just get a timeskip and fix their lives to come home to each other