I believe that there is a negative belief, or perhaps lots of negative beliefs, that you need to work on, and would recommend Bashar's method to clear them:

When you come up with that bad feeling that tells you apparently there the fire is dying down, be it in your imagination or a physical situation, make a note of the feelings you are experiencing in a lot of detail. Then ask yourself: What would I have to believe to feel this way? Insist on something until you get a sense of relief and truth from what comes up. When the belief comes up and it is negative, it will be so obviously nonsensical when it is on the surface of your mind that your mind will immediately reject it and it will be gone- it is only when a negative belief is unconscious that your mind will continue to hold it even when it does not serve you.

Repeat this process often.

Further, it will help to do the rest of Bashar's formula: Follow your highest excitement, to the best of your ability, without an expectation of a particular outcome. Doing this will, in practical terms, most likely nudge you towards certain changes that will do any number of these things: Making you more attractive, more relaxed, more friendly, more charismatic, more exciting, more in tune with compatible places and friends, more accomplished, more abundant and simply more likely to be in a reality that is genuinely positive- which is a really great way to start a relationship.

Another process I use is the Garden Tiki from Huna. Every morning and every evening (or at some kind of similar interval, a dozen times a day for a short period also works, for example) imagine that you are entering an inner place that is your own personal garden. Take whatever comes up- it's yours. Use a particular way of entering it- climbing up a certain staircase, coming down in a hot air balloon, whatever entry is attractive to you. Then look at your garden and experience it.

Once you get a feel for it, find things that can be improved- soil quality, patches that aren't clean, and, especially, weeds. The weeds can be anything that grows there that you don't approve of. When the weeds are gone, plant things there that you do like. Assume you have any help you need- little minions, dwarves, fairies, spirits, and even technology like huge earth moving devices or rototillers- whatever works for you. The important thing is if unpleasant things seem difficult to change- imagine something that makes it easy to change, and change it. In other words, insist on positive change, but use imagination to achieve it, not force.

When you feel noticable shift and it's time to move on, exit the garden the same way you entered it- go down the stairs, get into the hot air balloon and drift off until you wake up. This marks the closure of the process and you will certainly feel whatever changes you made. Results take time to build but the feeling changes will most likely be marked.

If you feel attracted to this permission slip for change, do use it. I took a little getting used to and needed to use Bashar's method to clear a belief that gardens represent unpleasant labor and no rewards before getting a knack of it, but it's a really nice form of symbol healing because you get to re-use the same symbol over and over again for all kinds of changes and that really adds to its power.

All the best, and Aloha!

The letting go part should not even be a vibration you have- you should just on a basic level know that whatever happens is for the best- loving yourself comes first- and other people are free to do whatever they want AS are you! Coming into a relationship with someone already having a thought like "I shouldnt get too attached because I have to feel like I should be totally fine with letting them go- THAT is your dominant vibration and im pretty sure you have witnessed first hand how that goes...of course everyone cam do and be as they please, if you love someone you have to understand that letting go CAN be one of the things we have to sometimes do in our life...but that does not mean not to still love them, care for them, think about them and do all those yummy delicious things you do with a lover...you should not think "omygod if theyll leave ILL DIE!!!" THAT means that you are too attached and then it will play out exactly like you are expecting

This is the reason that most relationships I am attached on dwindles down, the fire fizzles as soon as I get emotionally attached, regardless of whether or not I show what I feel or hide it.

I feel you on this one, I used to be exactly like this! It might be relevant for you to read the answer to another question here on IQ, where I told the story of manifesting a relationship with my now-boyfriend. Basically, we were casually seeing each other for half a year before he committed to a relationship, and in that half year, I had to work hard to let go of my attachment to him, and just let the relationship be what it was. And it was hard, because I was head over heels for him - butterflies in my stomach, getting excited over the simplest text messages from him, obsessing over him and stalking him on Facebook, etc. I had it BAD.

Link to my answer: HERE

As for how to let go of attachment, well it's not easy, but for me it was all about keeping my ideal relationship in mind, and knowing that I would have it, whether it was this guy or someone else. So it really boils down to trusting that the Universe will give you what you want. Your current crush could be "the one", or they could be a precursor to something even better.

Daydreaming in itself won't stop you from manifesting your relationship - lord knows I daydreamed a lot about my boyfriend. But the desperation will. So you need to let go of the attachment to this specific person. A good mantra to keep in mind when manifesting is: "this, or something even better".

You may also find this question that I asked many years ago about the same topic to be of help.

Good luck :)