Sounds as if your mentor is taking all the fun & joy out of quilting. She's expecting you to sew like she does even though she has 20 years up on you. If you are going to pull the borders off because of a small mistake you'll take a long time to finish one quilt.
"If a blind man on a galloping horse can't see it neither can anyone else.

I hope this little poem may help you:-

Your own ideas are as good as anyone else's
Make yourself happy by making the quilt you want.

I've adapted that from a garden sign (seen when BFF & I went to the Carnival of Flowers in Toowoomba).
I had permission from the home owner to adapt & use it.

You met your "quilting mentor" through your Guild, right? I'd suggest now is a good time to get more involved in your Guild with other members--think you will find that some will agree with ideas of your mentor, so will heartily disagree and you will actually advance as a quilter by being surrounded by so many different quilters. PLUS you will get to know some great people and have more fun with quilting. You might come to agree with some of your mentor's ideas--but at least it will be after you've heard/seen lots of others ideas, tried out some different ones on your own quilts and developed your own style. And get at least one quilt done and show at the Guild Show and Tell--think you will be affirmed that you are becoming a good quilter!

Whenever I mentor someone on quilting, I ALWAYS show them how I do things, but stress BIG time if they find a better way that works for them then go for it. Every quilter has their own way of doing things. There is no right or wrong way to quilt. It is all about what makes you happy & is easyer for you. We all here has faith in you to do great on your boarders, so go for it.

Oh, my goodness! I, too, am somewhat of a perfectionist, but have learned many times that the saying

'Done is better than perfect."

applies to all aspects of my life, including quilting. It's a humbling hobby, to say the least.

Note that I got over all my quilting phobias when I noticed that a picture of a quilt in a well known quilting magazine had unmatched points! :thumbup: That gave me the freedom to make mistakes!!!

You're a beginner, so have fun, experiment, enjoy the process as much as the finished product. Always keep in the front of your mind that quilting is not a life and death experience. Make mistakes, fix them. My sister and I both have commented that we have made most of our quilts twice: once the wrong way and once the right way. Such is life. That's how we learn.

Sewing two borders together before adding them to the quilt top is ok, depending on what you have in mind for the finished look of the border, although it does increase the possibility of wavy, hard to quilt borders. (Ask me how I know!) But, If it doesn't work out, well, that's why seam rippers were invented. Having said that, I recommend that you add them one at a time and follow your friends directions on how to do them, because it is good to know the technique, even if you don't use it after the first set of borders you add.

I use polyester thread all the time. So far, no one has been brave enough to complain to me about it. Perhaps my angry face would appear if they did!

I LOVE my BROTHER sewing machines! They are work horses. My go-to machine can sew fleece without any special attachments or ball point needles, and does a nice job of it. I found that out because I was too lazy to change the needle. :D

I used to write user manuals for business computer systems,and I can honestly say that Brother machines have the best user manuals I have ever seen/used, bar none - even the ones I wrote.

The best sewing machine for anyone is the one that is both affordable and enjoyable to use.

Your friend means well, but controlling is probably a good description. Forgive her; she can't help it. Then broaden your circle of quilting friends.

Everyone has said what I'd say about your mentor, enjoying the process and so on.
As to you having a Brother machine...hey, if it works for you and you like it...the heck with what other people think!! The best story I have about this is when the Executive Officer on my husband's ship (DH was a lowly commissioned warrant officer) came home early. I had a Featherweight and his wife, of course, had a mega-machine. His comment was, "What will your machine be when it grows up?" What a twit. My Featherweight is still going strong, but I bet her machine isn't. And why was he home early, anyway? So if you like it, use it and enjoy. You'll know when you want a different machine.

rules? I didn't know there wee rules and neither did all the quilters of my mother's and grandmother's generations. Are you planning on entering competitions? If not I suggest you break all the rules and be creative and do what pleases you. Look at all the wonderful scrappy quilters and the historic Alabama GEE quilters and as if they really cared about rules. Only rule I have is to have fun with the fabric!

you need to see all our first quilts! practice makes better. stop being so hard on yourself YOU MADE A QUILT! HOORAY! Now, some tricks. If you are concerned about seeing a 'mistake', make your border pieced. Large piano keys, squares, HST if you are feeling adventurous. You can break the eye with a small (1") solid border and then a larger pieced border. This will hide any little wobbles or weebles when you add the border. Be careful not to stretch either fabric. I am not much of a pin person, but for a beginner it would probably be very helpful. Also using a walking foot is very helpful. But, most important, stop being so hard on yourself.

I have a couple of adds....be sure your top is square before applying borders....that will take you a long way toward nice fitting borders. Trim up if you need to. Also, starch, starch, starch your borders and top to stabilize everything and minimize any stretching, etc.
And now to be the devils advocate....this woman who so willingly helped the OP may be a perfectionist and may be OCD ( I should know, I'm both) but y'all have sliced her up one way, down the next and sideways. Jeesh, some of it has been down right nasty. We don't know this poor quilter and while she may be making some mistakes in her tutoring, hasn't everyone who posted admitted they've made mistakes while quilting? Let's be a little more charitable and compassionate for someone who helped another learn to quilt...no matter what the "quality" of her teaching, lest ye be called the quilt teacher police! ;)

I have a Brother 650Q quilting machine. There is nothing wrong with your machine. Others will tell you theirs is better, and it may be, but your machine will do whatever you need it to, so buying a different one should be personal preference not because someone said to. Your machine will last you years as long as you keep it cleaned out, particularly under the foot feed plate. Clean it at least once a month or more often.