This is a familiar question/problem. The issue here is usually being uncomfortable with others around your front wheel and bars. I call this the "Sphere" as the old timers in the forum will know. I've posted about this here before but I think one post on red*** I've done explains it well. I use the same username on red***.
https://www.red***.com/r/Velo/commen...flowy_courses/
*edit: I suppose I can c/p it since I wrote it.
1. Reduce Sphere.
2. Learn to take wheels.
3. Move up when it's single file or spread out.
Whenever I hear of a rider that has problems surfing the front of the field, one scene pops up immediately. It was a Junior in a Cat 3 race. He was strong but he stayed a few feet off the wheel in front of him. In that Cat 3 race were a number of Masters National Champions (former and present), former Olympians, former Cat 1s, and probably a total of 30 national jerseys spread across 5 or 6 shoulders. Plus the regular aggressive 3s and former 2s.
I watched as the hapless Junior would move up hard on the outside, move in, and move back so quickly it was like his brakes were one.
At one point he cried out, "Why is this happening to me!?"
It must have been incredibly frustrating.
The problem was that he couldn't get too close to the wheel in front. He felt uncomfortable when someone moved into that "close area". I call that area your Sphere, the "personal area" around the handlebars that you need clear to feel comfortable. His Sphere was huge, probably 5 feet in front and maybe a foot to the sides. In a tight, experienced Cat 3 field that size Sphere was an invitation to move in, as if the Junior was backing off the let someone in and waving his hand to encourage the movement.
In contrast my Sphere is probably 2-3" to the sides and maybe 6" up front, meaning I'm not panicking if someone is that close. I prefer not to have any contact so I'll say 1" to the side and 2" up front for close quarters work, and I generally stay much further away than that, maybe a foot to the sides and a foot up front.
(All the above numbers are in the middle of the field where I don't know the riders well. If in a tight paceline with known riders then it's more like 1-2" up front - I'm okay with a lot of overlap - and next to known quantities I'm comfortable with side/shoulder contact for a second or two at a time.)
So first, reduce your Sphere. Do so by practicing bumping side to side, which reduces your side to side Sphere. Practice touching your front wheel into another rider's rear wheel (I detail elsewhere in r/Velo), keeping in mind that you will fall and therefore you need to be prepared. I did drills on grass/lawn wearing heavy clothing and keeping speeds down to a minimum.
Second, learn to take wheels. It is infinitely easier to take a wheel than defend one. Therefore do not try to defend wheels as much as take them. I can defend a wheel pretty ferociously but it requires very dirty riding and I'll lose the wheel in a minute or two of very intense battle, and probably make a lot of enemies doing it. In contrast I can take a wheel in about 15 seconds, gently, smoothly, with no choice on the other rider's part. Learn to take wheels rather than trying to defend your spot.
The trick to taking wheels is to back into the spot. If you want to take Rider A's wheel, get up sort of next to or slightly behind him, then move over a bit. Start with your knuckles about 1" from A's hip (small Sphere makes this possible). Then sort of drift back a bit while your knuckles move so that they're close to his rear wheel plane. You've only moved over about 6" but there's virtually no way someone can keep you off A's wheel. This is one of the absolute secret to racing effectively. It's mindblowing how effective this move is, how impossible it is to defend against it, etc. I have a couple hundred pages of racing tactic stuff written down but if I condensed it to one page this concept would stay on it.
When I run into someone that knows this then I just take the wheel back. Or, more often, I wait, because it's almost never so critical to have that wheel. It takes so little time to take the wheel back that you can put it off for 100 or 300 meters on the last lap, or for 5 laps if it's 5.5 laps to go. Etc.
Third, move up when it's strung out or starting to get spread out. The former requires some solid fitness. The latter requires having a smaller Sphere than those around you.