We've just been through exactly this. 2 months of waking every single 90 minutes. In the end, I contacted Andrea Grace after reading an MN thread about her. First night, Ds went a 6 hour stretch and then woke. By night 3, and ever since, he has slept through.
To be honest, she didn't tell me anything I didn't know, but I really felt I couldn't have done it without her.
Broadly, she recommended following the same pattern every night:
Bath, dressed for bed in our room, feed in his room with the lights on (preventing him from falling asleep on the breast), story (same story every night), say night night to lots of things (repeating phrases) then into the cot with the lights out. I then stayed with him til he slept. He cried, obviously. But I could stroke, pat, snuggle, kiss him, talk to him. Could even lift him out if I couldn't bear it (as long as I don't let him fall asleep on me and put him down awake). He fell asleep after 25 mins. Once he was asleep for the first time, then I wasn't to lift him out til 6am, but when he woke I could still pat and talk and stroke his back etc.
The plan has the parent gradually withdrawing over a week, so that in the end you put the baby down and they fall asleep happily, and sleep through.
So - it was really hard. And if you are against controlled crying then I think this will still feel too much. But - it worked and was much less traumatic than waiting outside as the baby screams.
Also - my baby was used to me feeding him to sleep - but he is big, so I felt confident he didn't need the food. If your baby is waking for different reasons, you may need a different plan.
Happy to discuss, obvs.