Please help, worming

Hi, I was just wondering what the best wormer would be to use on a nervous, unbroken mare around 4 years old, no previous worming has been done except one dose of an ivermectin based paste. She is barely handled and has a dull coat and a worm belly put will not gain weight. There is also a possibility of pregnancy as stallions got loose with her at the start of the year?
She was wormed last October, she has been left out to graze and did have bot eggs on her coat last summer and possibly this summer. She is being kept under awkward circumstances and I know she is most probably infested with worms. I am just lost on which will be the most effective for her.

In such a wormy horse, as you make her out to be, I would include the vet. If you dose too strongly, it may result in impaction when she tries to pass them. Phone calls are free, and he can tell you what you should start on. Perhaps some Strongid daily dewormer would be good, but I wouldn't start it without talking to your vet first.

I was thinking the same about the vet, i'll probably ring tomorrow. Thank you :)