I've been playing this game with some breaks since early in the beta when Chapter 5 and 6 were released. So about 8 or 9 months back I just completely lost interest even though I had just bought AP3. (No I don't regret that, at least I helped Gaz once again) I used to play this game like 8 hours a day, I loved it and I still do to be honest. No other game has took this much time from me. And I don't regret any of that time, aside from the times I spent costume rolling and inventory managing, those bored me a lot. :P But yeah, 8-9 months break, only played when new heroes were released. And that only lasted a few hours each time.
But last week I thought about giving it another try. And something magical happened. I was enjoying the game again. Now I don't think I can go for 8 hours a day like I used to but that doesn't mean I can't enjoy the few hours I play. Now I'm just gonna list the stuff that I think helped me enjoy the game again because I love making lists. (0 people were shocked by that, I'm sure)
Don't be too bothered by having amazing gear and just play. As far as I know, the game really isn't that challenging anymore. (AXIS raid was kinda hard when it first came out but I'm sure that's been dealt with) Sure, if that is what you want, do try to min/max stuff but I realized when I started not being bothered by that and enjoy the game casually, I certainly started having more fun.
Take a break. Seems like the most obvious response. A week, a month, a year. Take your time. Everyone needs a different time period for a break. Mine took me 8-9 months like I mentioned but now I think it's time to come back. Also, I'd recommend still hanging around in the forum. The devs, the community, the forum itself is awesome and very fun to hang with. Yes, I did take a several months off the forum but that's for different reasons that was necessary but let's not talk about that.
Your most wanted hero isn't everything. Believe me, at some point, no one's wish for their favourite character was even close to how much I wanted Wasp in this game. I was obsessed. So much that I took help. I'm sure no one else is nearly obsessed as I once was so I don't think getting help will be necesary but putting yourself into a mindset that "not worrying about if and when your favourite hero becomes playable will help you enjoy the game more" will definitely help you enjoy more. Believe me.
Read other people's experiences. This might sound wierd and it might only be me but whether it's in the forum or in-game chat, it always gets me hyped to read other people enjoying the game. Gives me the hope and certainity that I also will enjoy it. For example there is a Mr. Fantastic thread where the OP didn't enjoy him at all currently. And there were some people who did enjoy him so I got him, totally not that well geared and played him for over half an hour. It was fun! Got through cosmic trial with him!
So yeah, I will update this if more stuff comes to mind. I hope I managed to help you a little because I see a lot of veterans around losing interest which is to be expected. Just don't forget your interest won't be gone forever. Try to have it so that, it comes back while the game is still around. 2019 or further, this game will be gone one day. Enjoy it while you can.
Thanks for reading!