I don’t think you need to pretend like you don’t love him. It’s very healthy to go through the feelings of losing someone, acknowledging the love you have for them, and taking it day by day to find your new normal. Being vulnerable is okay, especially with yourself and your feelings.
if you see your ex as someone who you can eventually return to (which I think is totally possible) then maybe you can focus on identifying triggers within your relationship that contributed to yours and his mental health issues. Even if the relationship wasn’t the main cause of the deteriorating mental health, there are certain catalysts that can spiral you or your loved one downward if the triggers are happening on a day to day basis. In my experience, communication was what was missing. We were stressed in our own lives, and coming together didn’t exactly help when we couldn’t properly communicate what we needed from one another, whether that be support, consolation, or advice. Sometimes you lose yourself in a relationship, and that could be what he’s feeling. Maybe he just needs time to get back on his personal track before he can steer the relationship in the right way.
Regardless of what happens, I have some advice. if there’s any consolation, you can think about relationships like this: every person you meet means you’re one step closer to who you will end up with. With every relationship comes different traits from that person, things you like and things you don’t like. After any relationship, it’s important to identify those, so when moving forward you can steer clear from what you don’t like and pursue the traits you liked in your person. For instance, with my ex, I loved how positive he was and how he wanted to travel, but I didn’t like how he didn’t think logically about things and how I felt I constantly needed to rationalize with him. He was always fun and made every situation one to remember, but he was unorganized and scatterbrained. Going forward, I found someone who I can have fun with, who can rationally identify positives and negatives, who wants to travel, teaches me something new everyday, and is one of the most organized and well thought out people I know. No love is like the other, but you can find things that made you love someone and THEN some new traits in another person.
… or you can take time, let them grow, let yourself grow, and come back to see if some issues were naturally fixed.
Best of luck to you!