She's very lame, as others have said, both front and behind. I would retire her and let her live her days being happy as a companion
in the field. I wouldn't use her for anything, other than a companion.
I'm in your area OP. (I even recognise the school she is in as venue I compete at!) if you want some help to find something more suitable (forgive me if I'm wrong but you sound quite inexperienced?) I'm happy to keep an eye out for you. I'm certainly no expert but know a lot of horsey people in the area who might have something suitable. PM me if I can help. Are you looking loan, buy or share?
I'm sorry OP but I can't see this horse having a ridden future. If she's like that after having months of field rest, then with riding she would only get worse. You would only be buying yourself heartache. She's lame behind - typical toe dragging associated with kissing spine/SI problems. Also, she's very lame in front. I think that it's actually cruel for this horse to be ridden in this state. Having said that, she looks like a lovely little mare - what an angel!
That poor, sweet and lovely mare needs a lot of vet attention. She is very, very lame. OP if you have oodles of money and don't mind having a field ornament then get her. Just prepare yourself for the worst - that wasn't just stiffness. She will take a lot of time and money to get her right, if she can ever be got right at all.
From looking at the clip, I cannot tell whether the lameness is mechanical or pain related. The horse looks extremely relaxed with virtually no tail swishing, so I don't think she looks in a great deal of pain to me. Having said that, the lameness does seem to be coming from higher up (back/SI region), and the video is too poor for me to see her expression. Personally, I think this lovely kind natured mare should be retired to the field.