Hey! I really suffered mentally too. I injured myself in August 2019. After getting down to like 15% body fat and the fittest I have ever been. I didn’t have surgery until 10 days ago. All I can say is this - there will be a recovery afterwards we’re you can work on the fitness and weight afterwards! I found it a lonely time pre operation so I did what I could with friends and family which usually meant eating. If you can see if you can use a static bike until your operation. Ask what you may be able to do. I know I’ve put on a good stone or so and many inches I’m gutted but at the same time I wasn’t moving and sometimes sitting eating junk watching a film brought great happiness.
Don’t beat yourself up about it. Stay aware and focused on your end goal and whatever your advised exercise Wise try it and keep going xx best of luck.
I feel your pain!! I did my acl 15 days ago. The first week was rough, between coming to terms with the injury and my functionality was playing havoc on my mental health (never in my life thought I’d ever say that! Im normally a strong/bubbly person) after the first week/during the second week it got better though. Moving into wk 3 and I saw a specilist today and got told im going for hamstrung graft as soon as the swelling subsides although I wanted to go for a different procedure thats not yet available in Australia 
key points ive gathered so far - prehab, prehab, prehab! But don’t over do it. I made drastic improvements over 7 days to being almost back to normal function (just no running/sports as such) doctor said im fine to get back on my bike though! So adventure can still be had, just not in the same ways as before and it now takes caution.