It's a classic negotiation opportunity, rather than a hair-tearing moment. When is your room getting measured? Has it already happened, and if so, did your mum approve a quote or sign a sales contract?
You need calmer analysis as a start-point for negotiation.
The new storage is going to happen, but there's still time for you to extract benefit from the situation. There may be some flexibility on:
1. The storage type: does it really need to be hanging space, or would a raised bed (or platform bed) with storage below offer an acceptable/better/cheaper/preferable alternative solution?
2. The storage location: perhaps the 'expert' who measures up may have a more tolerable suggestion for the placement of the wardrobes in your room, which could minimise the disruption of your space
3. The storage supplier: can you research/identify a company whose products are more acceptable to you in terms of their design?
4. The storage specification: is it an 'off the peg' type of standardised product, or made to measure/by a carpenter (who could possibly integrate your bookshelf or a bookshelf)
5. The allocation of utility of the storage: how much of it is for your own use?
6. Is there any other change in your room that you would like to happen: new paint job?
7. Is there any other enticement/privilege that you would like to happen (unrelated to your bedroom) that you could negotiate for, in exchange for embracing the installation of storage space?
8. Is there another family member who would swap rooms? If your room is no longer likely to feel your 'own', then this could be an opportunity to do this.
9. Can your uncle's bookshelf be retained and re-installed elsewhere in the house?
10. Could your uncle, or someone equally skilled, make you a new bookshelf for your redesigned room?
Mature negotiation is always more constructive than blocking meltdowns. Failing all of that, just try to get some perspective: one of my teenagers slept outside in a carpark last night, to get some insight into homelessness. All in all, life is good if you're just stressing about fitted wardrobes.