Firstly, very few people are happy with their own portfolio. Imposter syndrome is understandable and common, but if you've put the time in to your drawing over the years you've got nothing to worry about. You will find a course that you can thrive in!
If possible, attend Open Days of the places you'd like to apply to. Then ask what they want to see in a portfolio, and how to present it.
Most courses will be fine with Google Slides, but some may ask to see a physical portfolio if you're invited in to an interview.
You can get a reference from teachers at your last place of education. Just email them in advance to ask if it's ok.