Righto, here's my perspective...
Hey fella.
First of all, sorry to hear that you're in this position. I'm not gonna sugar coat it, you're not going to have much fun, but it will always get better.
I'll echo what our riding comrades have said and i'll add my perspective from the point of view of a three time ACL reconstructed individual. One that has just left the surgeon's rooms post 4 month review.
Right, first of all. Do you know who your surgeon is? How many opinions have you had? It may be too late for this, but if you've only seen one person, and there's time, see another. Yes, this type of surgery is performed all the time, but as they say, oils ain't oils.... If you're locked in, that's fine, go with it.
So, immediately post-surgery you're probably gonna feel pretty good, but as the days go on the inflammation increases, stiffness sets in and you won't shit for days... thanks to the pain killers and the drugs your friendly anaesthetist will give you. Get some prune juice into ya, just don't use any bulking agents, such as Psyllium, they'll only slow things down. Best thing you can do to get through this stage is really focus on something, usually in this early stage it will be getting your leg straight again, it sounds trivial, but it's really important. Secondly, ice, ice and MORE ICE!!! Get a routine going, every moment possible, ice that knee and keep it elevated, trust me it will be worth it.
Get out of the house as much as possible. You'll be surprised by how much you can get about, but don't push it too much. That little bit of fresh air each day will do wonders for your body, and your mind.
As time progresses keep your focus on your rehab exercises and reducing swelling, whilst increasing mobility. Every little gain will feel amazing, but be prepared for little set backs. They happen, it's fine, just don't dwell on them.
The more mobile you get the more you will long for the things you can't do. I sold my mountain bike. It was a sense of strike while the iron is hot, and when i could get the most cash for it. I already had a road bike. I'd suggest getting a cheap roadie or hardtail, you might even be able to pedal around the block after about 4 weeks, but it may be closer to 8, everyone is different.
Keep your head involved with bikes, you'll be surprised that it won't be long till you're back on one, even if in the short term it is only at the gym. Believe me though, even that can feel amazing!!
Biking is my outlet too, and i've very much missed it. Focus on small goals on the stationery bike, and then as you progress on the road/hardtail. Those small goals will keep your head feeling good, soon those small goals will become big goals and so on...
Ultimately the recovery is more of a mental journey than a physical one. The healing/graft side of it has it's on pace, and it can't be sped up, don't forget that. Do anything you can to stay engaged with your rehab program. Find a good SPORTS physio, shop around, stick with one you're happy with. They can keep it fresh, but they're also a great outlet to voice your concerns and frustrations, believe me, they get it.
To sum it up. Everyone is different, we all heal different and no two surgeries are the same. Listen to your body, listen to your mind, and you'll get through it!!
Just in case you're wondering, THREE ACL reconstructions... yep, three.... I had a bad initial injury that was poorly treated and basically set the wheels in motion of 15+ years of surgery and rehab. I've seen a lot in that time in regards to surgical approaches, research and rehab, I also work in the surgical field myself, so I feel pretty confident in offering my perspective. But, it is only that, my perspective.
Anyway, i've rambled enough. Hopefully you can pull something useful out of that. Always happy to chat/shoot the shit if need.
Cheers, Grant.