Applying for TN Visa - Nervous about getting declined

This past year I have been visiting my girlfriend in the US (Texas). I've gone there a total of 4 times this year, each with a span of 1 to 1 1/2 weeks.
My previous visit the customs officer grilled me asking what my purpose was and I said i was visting friends. Then she asked, is your friend a girl, met them through high school, etc. I said i'm just visiting friends for thanksgiving. She then seen my promise ring on my finger and asked if why my wife wasn't with me. I says, it's a promise ring not a wedding ring, then she asked why my gf wasn't coming with me. I basically told the customs officer that my gf couldn't come.

My work has sponsered me to aquire my TN Visa in January and work for their US Branch. With these questions that from my previous trip, it has really gotten me scared and nervous that I will be denied entry. My employment letter has all the necessary things needed, stating my stay is temporary, my job description, etc.

Any advice to bring my confidence back up when I apply in January? I know they will be asking me why I've been to Texas so much in the past year.

You're better off being honest with the customs people. There is nothing wrong with having a g/f in the U.S. - she's not your fiancee or wife, so there shouldn't be a problem with them thinking you're immigrating. My b/f is an American citizen and drove me through the border when I got my TN. They did ask who he was to me, when we met, how we met, etc, and they will ask you questions at a very rapid rate. Be honest and truthful (yet you may not want to say you're planning on immigrating). If you hesitate with your answers or can't answer, that's when they get suspicious.

As well with visiting the U.S., I always said I was visiting my boyfriend and never once had a problem.

And yet always always always be aware that having a boyfriend or girlfriend of any sort in the US is grounds for a US CBP to deny your entry, or to deny a TN, if they think you are, or will be intending to immigrate from a non-immigrant status.

Sure, a few get thru with no problems, and quite a lot more get turned away every day. Depends on who you get, and their mood when you get there.

I was lectured at length by a USCIS agent at Sweetgrass who told me it was perfectly okay to have a girlfriend in the US who I was visiting, in fact she wouldn't believe I wasn't going to visit a girlfriend and kept telling me I wouldn't be denied entry if I admitted it!

If you go in as TN-1 you have the problem of maintaining "bona fide non-immigrant intent". Not a problem as long as she remains a girlfriend, if you get engaged then it will get sticky, because USCIS usually won't believe you don't have the intent of staying permanently. At that point it's better to simply get married and apply for AOS.