So frustrating when you're in that cycle isn't it. Bet there's not many on minis who haven't been there.
What exactly happens by day 3? When do you find it hard to stay on plan - is it the same situations, people, habits. Once you recognise your triggers you can have a plan to break them. You can overcome most things with a bit of planning.
Keeping a diary helps me. Write everything down. I find this keeps me accountable and I feel more in control. Even if I have a bad day I syn everything. Even if I go over, by writing it down I'm eating more consciously and It helps me stay on plan despite a bad day.
Do you plan out your week and shop ahead? Not like a regimented menu but have a few days worth of SW friendly breakfasts, snacks, lunches and dinners on hand so it's easier to make good choices. Maybe take some time in the evening to think about the next day. Where will the challenges be, how you'll get through them. If you're not sure how to get past them then post whatever it is on here and others will tell you what works for them.
Try to do SW one day at a time. Set yourself small achievable goals and be proud of yourself when you achieve them. Share them here, you'll get loads of encouragement. Use your syns to have some of the things you enjoy so you don't feel deprived. And with SW you don't ever need to feel hungry.
Try to really focus on why losing weight is important to you. Feeling like you "have to" can be demotivating (you mention health reasons). Try to remember all the reasons you really want to. Motivation is so personal but remember to do it for you.
Don't give up!