A couple of things that have helped me.

Fitting for putter, I needed a shorter putter for correct eye alignment and was better with a strong arc blade putter.

Develop a pre-shot routine. Ex. 1) count steps to the hole and walk around the pen to feel for slope 2) notice that dark grass means slower putts, light grass is faster. 3) Find a spot inline to the hole based on the break you think there is, do not aim directly at the hole 4) relax, take a deep breath and do not over grip the putter 5) take a few practice stokes from soft to harder, when it feels to hard go back to last speed 6) aim for the spot you found in # 3 and putt.

My first putting lesson was to putt to a target 20 steps away. Goal was not to go in hole but get about 12 inches past the hole every time.

Before a round I do a few putts to a hole or target 20 steps away on a level spot to get idea of speed. Then do 10 step putts and then 3 step putts for speed.

Be sure to stand with dominate eye over the ball for better alignment. A putting mirror may help.

Take a deep breathe to help relax. When you are tense your posture is out of alignment, tend to tighten shoulders and bend over too much.

Everyone misses putts, pros make about 75% of 5 foot putts, a golfer in the nineties make 50%. Don’t be too hard on yourself. Have fun.

As I am finishing my rehab I have started doing some putting. Not bad having been away since early December. Did realize that was not putting online and wasting too much time trying to set up my putter. Making sure that the toe was not too high, remembering what my instructor had said. We had had my other putter set at one degree flat. Took it to my favorite shop and they adjusted the putter and practiced with it. Do not have to worry now. Line it up and nice easy stroke and very much on line. Proper adjustments can make a difference also. When you have to think over a putt, a lot can go wrong.

I agree with DRoss. Get fitted for your putter. I had been buying off the rack and guessing what I needed. When I finally got serious and disgusted enough with missing 4 footers, I took my SC Concept X to a Titleist putter fitting and WOW... what a difference. As it turns out, the grip was too fat, length too long, and weight too heavy. I have it dialed in now and can't miss...

Now I use a long putter but when I get nervous I squeeze my left elbow to help focus on the ball and get out of trying to make the perfect stroke

The best place to get comfortable and familiar with your putter is on the practice green when you are not playing.
Spend some quality time on the practice greens working on the areas that are causing you the most problems.
If can develop speeds from distances, correct grip pressure, alignment, etc. Spending a few hours each week can save you a few strokes on the weekend in that most people work thru the week. Just sayin'.

Going back into when I use to play softball (26 years of league and travel). When not playing I would spend maybe three nights a week at a batting cage for about an hour hitting balls, working on ball placement and power. Usually lead my team in BA and HRs.

I agree with the advice for a pre shot routine. Whether in the putting, or swinging full clubs. A good pre shot takes away some variables and develops some confidence. You're still going to miss a lot of putts, but it helps to at least be comfortable over the ball because you know you're set up. I've got a Birdieball putting matt in the basement. It's repetitive, hitting the same putts over and over. But a few minutes each day and I'll feel better over the ball once the courses open up. That's going to be a while, though. Snowing here again today and a big ice fishing weekend coming up.

I agree that a pre shot routine is a good thing, and should really help. Keep in mind, generally speaking, that the longer you stand over a putt the more uncomfortable you'll become.