I had exactly the same injury, it first happened in the ring when I blocked a kick (obviously im a kickboxer not a boxer) and it bend my thumb right back. I had movement but it swelled up and hurt like hell for months. Everytime I thought it was getting better I would catch it either on the bag or sparring and it would be really painfull again. I think I just badly bruised the ligaments but the annoying thing was I would always catch it on things like closing the car door & stuff so it just took ages to heal because its hard to rest your fingers because you use them so much, I think it took a good year to heal properly but even now I still catch it now and then on the bag or sparring. I just try and keep it tucked in tight now when im punching.

Just give it some time and I am sure it will heal but if you are anything like me and find it hard to rest when you pick up injuries it may take some time.Good luck man!!